Method of controlling hydraulic pressure in speed change mechanism having hydraulic clutch

ABSTRACT

A speed change mechanism ( 1 ) constructed by connecting in tandem a hydraulic type speed change unit ( 17 ) having a plurality of hydraulic clutches ( 57, 58, 59 ) to be alternatively engaged and a hydraulic type speed change unit ( 20 ) having a plurality of hydraulic clutches ( 66, 67, 68 ) to be alternatively engaged, wherein a time-varying region (common slip region) is secured in which the two clutches slip in common such that during speed change, when the working hydraulic pressure in a clutch to be engaged is on its way to gradual increase, the working hydraulic pressure in a clutch to be disengaged lowers. This common slip region is made smaller during shift-down than during shift-up by a change in time-difference between the pressure increase start time for the clutch to be engaged and the pressure decrease start time for the clutch to be disengaged or by a change in the pressure decrease property of the clutch to be disengaged, and is maintained constant irrespective of whether one or two hydraulic clutches are engaged and disengaged during speed change respectively or irrespective of a difference in engine rpm.

TECHNICAL FIELD

The present invention relates to a method of controlling hydraulicpressures in a speed changing mechanism having a plurality of hydraulicclutches, that is, a hydraulic power shift speed change mechanism.Particularly, the invention relates to a method of controlling hydraulicpressures in a multistep-speed-type speed change mechanism constitutedsuch that a plurality of hydraulic type speed change units are connectedin tandem, wherein each of the hydraulic type speed change units isconstituted of a plurality of transmission trains, and a hydraulicclutch is provided in each of the transmission trains.

BACKGROUND ART

Conventionally, there is publicly known a so-called hydraulic powershift speed change mechanism configured of a plurality of hydraulicclutches (fluid-operated multidisc clutches). Particularly, there ispublicly known a multistep-speed-change-type speed change mechanismconstituted such that a plurality of hydraulic type speed change unitsare connected in tandem, wherein each of the hydraulic type speed changeunits is constituted of a plurality of transmission trains, and ahydraulic clutch is provided in each of the transmission trains. Invehicles including the speed change mechanism, such as an agriculturaland other work tractors, speed-changing for the number of steps obtainedby multiplying the numbers of transmission trains provided in individualspeed change units. Suppose a speed change mechanism configured of twohydraulic type speed change units, in which two transmission trains areprovided in one of the hydraulic type speed change units, and threetransmission trains are provided in the other hydraulic type speedchange unit. In this case, 2×3 steps are obtained; that is, totally,six-step speed changes can be performed.

Conventionally, to perform input/output control forengagement/disengagement operating fluid for individual hydraulicclutches in the above-described speed change mechanism,electromagnetic-type selector valves are used.

In connection with the conventional hydraulic-pressure control for thehydraulic clutches at the time of speed-changing, first of all, therelationship in time between engagement-objective clutches anddisengagement-objective clutches will be described below. Essentialthings regarding speed-changing include the prevention from a case wheredouble transmission trains are operated to be in transmission states.Specifically, in the above-described multistep-speed-change-type speedchange mechanism configured by combining the plurality of hydraulic typespeed change units, it is essential to avoid a case where two clutchesare operated in an engaged state in each of the speed change units.Therefore, conventionally, a disengagement-objective clutch is firstdisengaged substantially completely; and after a nontransmission stateis once made in the speed change mechanism, the engagement of theengagement-objective clutch is then started. However, a high load isimposed during a nontransmission state, the vehicle is likely stopped.In addition, since a hydraulic pressure begins to rise from thenontransmission state when the engagement-objective clutch startsengagement, there remain problems which cannot be solved in that greatshocks occur, thereby causing an operator to feel uncomfortable.

In view of the above, as described below in the “Disclosure ofInvention” and in other portions, even when the transmission efficiencyis reduced to the lowest level during speed-changing, at least eitherthe disengagement-objective clutches or the engagement-objectiveclutches are controlled to be in slip states. Specifically, operatingtiming and a time-transitional hydraulic pressure property for theindividual disengagement-objective clutch and the individualengagement-objective clutch are set so that a region representing a slipstate (the region will hereinbelow be referred to as a “common slipregion”) for the two clutches can be secured.

Hereinbelow, a brief description will be made regarding clutch hydraulicpressures in the slip state. The pressure for a disengaged clutch in afluid chamber is substantially 0, and a piston for operating a clutchdisc is in a free state. To engage the disengaged clutch, first, fluidis fed to a fluid chamber therefor to be filled out, and the filled outfluid must be used to increase the pressure to hold the piston. When ahydraulic pressure having a value that is sufficiently high to hold atleast the piston is set to a piston-holding pressure, the hydraulicpiston is brought to a slip state at an operating hydraulic pressurethat is higher than the piston-holding pressure.

However, different from the above-described conventionalhydraulic-pressure control for which the relationship between theindividual hydraulic pressure states for the disengagement-objectiveclutch and the engagement-objective clutch need not be taken intoaccount, in the hydraulic-pressure control of the present invention,when the individual time-transitional hydraulic pressure properties forthe engagement-objective clutch and the disengagement-objective clutchare fixed as have been set under specific conditions where, for example,the engine is operated at a rated revolution frequency, there occurscases wherein no common slip region can be secured because of theconditional variations.

For example, in a speed change mechanism configured of two hydraulictype speed change units, there are two speed-changes. One of the speedchanges is performed such that in one of the hydraulic type speed changeunits, clutches remain held in engaged states; and in the otherhydraulic type speed change unit, one engaged clutch is disengaged, anda different clutch is newly engaged (one-objective-based hydraulicclutches are disengaged/engaged). The other speed change is performedsuch that, in each of the hydraulic type speed change units, one engagedclutch is disengaged, and a different clutch is newly engaged; that is,in the overall speed change mechanism, totally, two clutches aredisengaged, and two clutches are engaged (two-objective-based hydraulicclutches are disengaged/engaged). As described above, before anengagement-objective clutch is controlled to be in a slip state, waittime is required until the pressure increases up to the level of thepiston-holding pressure after the fluid is injected into the fluidchamber of the clutch. For two-objective-based hydraulic clutches to bedisengaged/engaged, aforementioned time is required substantially twiceas much as that in the case where one-objective-based hydraulic clutchesare disengaged/engaged. Therefore, when clutch-timing and atime-transitional hydraulic pressure property are set to secure a commonslip region according to the case where the one-objective-basedhydraulic clutches are disengaged/engaged, they are not suitable to thecase where the two-objective-based hydraulic clutches aredisengaged/engaged.

When the engine revolution frequency is reduced, time required forfilling out the fluid in the clutch fluid chamber is increased.Therefore, for example, hydraulic-pressure control is set to obtain acommon slip region during a rated revolution. However, problems similarto the above can arise during idle revolution.

In comparison between a speed-changing operation at a shifting-up timeand a speed-changing operation at a shifting-down time, in the formercase, since the relative revolution speed of a secondary-side rotationshaft with respect to that on a primary side of an engaged/disengaged isincreased, a common-slip-region period needs to be set to be relativelylong. On the other hand, in the latter case, the speed-changing isperformed to reduce the relative revolution speed of the samesecondary-side rotation shaft, and rotational inertia at a time ofpreshift operation is imposed on the secondary-side rotation shaft.Therefore, the common-slip-region period may be short; and when it islong, smooth speed-changing is impaired.

As in the conventional case, in speed-changing in which anengagement-objective clutch is engaged after a disengagement-objectiveclutch is disengaged, detection is performed by using a pressure sensoror the like for the state of engagement of the disengagement-objectiveclutch that is supposed to have been engaged. Checking is therebyperformed for abnormality (such as entrance of foreign substances).Thereafter, engagement of the engagement-objective clutch isinterrupted, thereby allowing double transmission to be avoided. As inthe case of the present invention, in the speed-changing in which acommon slip region is secured, disengaging operations and engagingoperations of clutches are overlapped. Therefore, there can be caused acase where a disengagement-objective clutch is not disengaged, while anengagement-objective clutch is engaged. That is, there can be causeddouble transmission that can cause damage in the transmission mechanism.Therefore, an abnormality-detecting method suitable to the presentinvention is demanded.

Pressure-increase properties required for the engagement-objectiveclutches at the time of speed-changing are different depending on thetraveling mode of a work vehicle employing the speed change mechanism;that is, the properties differ depending on whether the vehicle isengaged in normal (on-the-road) traveling or tractional traveling. In atractional travel time, the hydraulic pressure at a rising time needs tobe set high, and the clutch needs to be quickly engaged. Otherwise, thetransmission efficiency is not sufficient to catch up with the load,thereby causing engine failure. To reduce shock that can be caused in anormal travel time, rising pressure is preferably controlled as low aspossible.

Conventionally, to overcome these problems, in a hydraulic-pressurecontrol system for hydraulic clutches, two types of pressure-increaseproperties, one for normal traveling and another for tractionaltraveling so as to be alternatively selected by an operator are stored.

However, problems still remain pending resolution. With a control methodthat is dependent on operator's switching operation, when erroneousoperation is performed, there occurs hydraulic-pressure increase thatdoes not correspond to practical requirements, causing problems such asengine failure and shock generation. To cope with these problems, thecontrol is preferably arranged such that the load state is automaticallycan be detected, and one of the hydraulic-pressure-increase propertiescan be selected according to the result of the detection.

In addition, as described above, the variety of conditions varies therequirements regarding, for example, hydraulic-clutchengagement/disengagement operations at the time of speed-changing, i.e.,the time-transitional hydraulic-pressure-increase properties forengagement-objective clutches, time-transitionalhydraulic-pressure-decrease properties, and the operational timing. Tocomply with these requirements, it is preferable that input/outputhydraulic pressures for clutches be controlled to be variable; that is,it is preferable that the capacity of an individual clutch-operatingvalve be variable.

DISCLOSURE OF THE INVENTION

The present invention relates to a speed change mechanism (so-calledhydraulic power shift speed change mechanism) having a plurality ofspeed-changing hydraulic clutches, each of which is engaged according tohydraulic-pressure-increase effects and is disengaged according tohydraulic-pressure-decrease effects. A primary object of the inventionis to avoid a nontransmission state that can occur at a time ofspeed-changing with the speed change mechanism.

To achieve the object, according to the present invention, at a time ofspeed-changing operation, an operating hydraulic pressure for a clutchto be engaged from a disengaged state is gradually increased in a timetransition, and an operating hydraulic pressure for the clutch to bedisengaged from an engaged state is reduced during the gradual pressureincrease. Preferably, during the speed-changing operation, anoperating-hydraulic-pressure-decrease start time for thedisengagement-objective clutch is set to be later than anoperating-hydraulic-pressure-increase start time at which a fluidchamber of the engagement-objective clutch becomes full of fluid, andthe pressure thereof rises to a piston-holding pressure. Thereby, atime-transitional pressure region (common slip region) where anengagement-objective clutch and a disengagement-objective clutchcommonly slip at the time of speed-changing operation is secured.

Also, in connection with the aforementioned object, in order to allowthe common slip region to be constantly secured at all times regardlessof various conditional variations, at least one of a time differencebetween the operating-hydraulic-pressure-increase start time for theengagement-objective clutch and theoperating-hydraulic-pressure-decrease start time for thedisengagement-objective clutch and a time-transitional decrease propertyin the operating pressure for the disengagement-objective clutch iscontrolled to vary corresponding to engine revolution frequencies.

In this case, the various conditions include engine revolutionfrequency. Corresponding to the property that a fluid-chamberfilling-out time for the engagement-objective clutch increases inproportion to reduction in the engine revolution frequency, when thetime difference is controlled to vary, the aforementioned timedifference is set longer in proportion to reduction in the enginerevolution frequency or in a case where the engine revolution frequencyis equal to or lower than a specific level so as to decrease slower inproportion to reduction in the engine revolution frequency or in a casewhere the engine revolution frequency is equal to or lower than aspecific level.

In the speed change mechanism (so-called multistep-speed-change-typespeed change mechanism) configured by classifying the aforementionedplurality of speed-changing hydraulic clutches to allocate them to aplurality of hydraulic type speed change units, the hydraulic clutchesare alternatively engaged in each of the hydraulic type speed changeunit to thereby form one speed step. In this configuration, as describedabove, in order to secure the time-transitional pressure region (commonslip region) where the engagement-objective clutch and thedisengagement-objective clutch at the time of speed-changing commonlyslip, when the hydraulic-pressure control in which the operatinghydraulic pressure for the clutch to be engaged from a disengaged stateis gradually increased in the time transition, and an operatinghydraulic pressure for the clutch to be disengaged from an engaged stateis reduced during the gradual pressure increase at the time ofspeed-changing is employed, the number of clutches to beengaged/disengaged is included as one of the aforementioned variousconditions. Therefore, when the time difference is controlled to vary,the time difference is set relatively long at a time of speed-changingwhen the number of the clutches to be engaged/disengaged is large, andthe time-transitional decrease property is controlled to vary, thetime-transitional decrease property is set to be reduced slower at atime of speed-changing when the number of the clutches to beengaged/disengaged is large.

Considering that a rotational inertia is imposed at a time ofshifting-down operation compared to a case at a time of the shifting-upoperation, in order to reduce the area of a common slip region at thetime of the shifting-down operation to be narrower than that at the timeof shifting-up operation, at least one of a time difference between theoperating-hydraulic-pressure-increase start time for theengagement-objective clutch and theoperating-hydraulic-pressure-decrease start time for thedisengagement-objective clutch and a time-transitional decrease propertyin the operating pressure for the disengagement-objective clutch iscontrolled to vary depending on whether the speed-changing operation isa shifting-up operation or a shifting-down operation. For example, thetime difference is set to be relatively short.

In this case, it is preferable that, during speed-changing, regardlessof variations in the time difference and the time-transitional decreaseproperty that have been set to meet the aforementioned individualconditions, the operating-hydraulic-pressure-decrease start time for thedisengagement-objective clutch be set to be later than theoperating-hydraulic-pressure-increase start time at which the fluidchamber of the engagement-objective clutch becomes full of fluid, andthe pressure thereof rises to the piston-holding pressure.

Another object of the present invention is to provide an appropriatemethod to detect an abnormal clutch to prevent the occurrence of adouble-transmission state in the hydraulic power shift speed changemechanism for which the hydraulic-pressure control is performed asdescribed above.

To achieve this object, a pressure-detecting means is provided to detectan operating hydraulic pressure for each of the hydraulic clutches, andwhen the number of the pressure-detecting means for detecting hydraulicpressures higher than a specific pressure value is greater than thenumber of the hydraulic clutches to be engaged at the time ofspeed-changing operation (in the speed change unit configured of theplurality of hydraulic type speed change units that are connected to intandem, when two or more units of the detecting means each detect apressure higher than a specific pressure value in at least in one of thehydraulic type speed change units), one of two hydraulic-pressurecontrol operations is performed, one hydraulic-pressure controloperation being performed to engage only those of the hydraulic clutcheswhich have immediate-previously been disengaged, and the other onehydraulic-pressure control operation being performed to disengage allthe hydraulic clutches.

The individual pressure-detecting means may be configured such that theindividual means constitute switches each turning ON or OFF with respectto the border of the specific pressure value, and when the number of thepressure-detecting means for detecting hydraulic pressures higher than aspecific pressure value is greater than the number of the hydraulicclutches to be engaged at the time of speed-changing operation (in thespeed change unit configured of the plurality of hydraulic type speedchange units that are connected to in tandem, when two or more units ofthe detecting means each detect a pressure higher than a specificpressure value in at least in one of the hydraulic type speed changeunits), one of two hydraulic-pressure control operations is performed,one hydraulic-pressure control operation being performed to engage onlythose of the hydraulic clutches which have immediate-previously beendisengaged, and the other one hydraulic-pressure control operation beingperformed to disengage all the hydraulic clutches.

Still another object of the present invention is to detect whether aload is imposed on a vehicle by using appropriate detecting means, notby relying on operator-performing switch operations. This allowsoperating hydraulic pressures for the individual hydraulic clutches tobe appropriately increased without failure.

To achieve this object, in the present invention tractional-loaddetecting means is provided in a vehicle employing the speed changemechanism to thereby modify a time-transitional increase property in theoperating pressure for the hydraulic clutch to be engaged at the time ofspeed-changing and a time-transitional decrease property in theoperating pressure for the hydraulic clutch to be disengaged at the timeof speed-changing depending on whether or not the tractional-loaddetecting means detects a tractional load. Alternatively, when agovernor mechanism capable of performing control of an engine revolutionfrequency according to detection of an engine load is provided in thevehicle employing the speed change mechanism, the governor is used tomodify a time-transitional increase property in the operating pressurefor the hydraulic clutch to be engaged at the time of speed-changingdepending on whether or not the governor mechanism detects an engineload equal to or higher than a specific level.

The above load detection may be used to modify a time-transitionaldecrease property in the operating pressure for the hydraulic clutch tobe disengaged at the time of speed-changing.

As summarized above, the speed change mechanism comprising hydraulicclutch according to the present invention, corresponding to the variousconditions modifies the time difference between theoperating-hydraulic-pressure-increase start time for theengagement-objective clutch, the operating-hydraulic-pressure-decreasestart time for the disengagement-objective clutch, and thetime-transitional decrease property in the operating pressure for thedisengagement-objective clutch at the time of speed-changing operation.Therefore, in order to allow input/output pressures of operating fluidfed to each of the hydraulic clutches to be adjustable, the individualhydraulic clutch is controlled by means of an electromagnetic pressureproportion valve provided for each of the hydraulic clutches.

The above and other objects, configurations, and advantages of theinvention will become apparent from the following detailed descriptionthereof taken in conjugation with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a skeletal view of a tractor transmission system having anine-step-variable-speed-type hydraulic primary speed change mechanism1;

FIG. 2 is a diagram of a hydraulic-clutch-controlling hydraulic circuitin the primary speed change mechanism 1;

FIG. 3 is a block diagram of an embodiment of an electrical controllercircuit in the primary speed change mechanism 1;

FIG. 4 is a block diagram of another embodiment of an electricalcontroller circuit;

FIG. 5 is a skeletal view of a tractor transmission system having asix-step-variable-speed-type hydraulic primary speed change mechanism1′;

FIG. 6 is a diagram of a hydraulic-clutch-controlling hydraulic circuitin the primary speed change mechanism 1′;

FIG. 7 is a side view of a tractor employing the electrical controllersystem shown in FIG. 4;

FIG. 8 is a plan view of the aforementioned tractor;

FIG. 9 is a plan view of a primary-speed change hydraulic valve unit 3for the primary speed change mechanism 1′;

FIG. 10 is a time-transitional hydraulic pressure graph showing apressure-increase property for an engagement-objective clutch;

FIG. 11 is a time-transitional hydraulic pressure graph showing apressure-decrease property for a disengagement-objective clutch;

FIG. 12 is a time-transitional voltage graph regarding left and rightdraft sensors 112R and 112L functioning as load-detecting means forpressure-increase-property determination;

FIG. 13 is a flowchart of pressure-increase-property determination usinga draft sensor 122 and a traction sensor 123 that are disposed on theleft and right sides;

FIG. 14 is a time-transitional graph of engine revolution frequenciesdetected for the pressure-increase-property determination;

FIG. 15 is a time-transitional graph of rack positions detected for thepressure-increase-property determination;

FIGS. 16A and 16B show a flowchart of the pressure-increase-propertydetermination using an electronic governor;

FIG. 17 shows time-transitional graphs regarding input voltages fromhydraulic-clutch-operating hydraulic pressures and individual pressuresensors in a first hydraulic type speed change unit 17′ and a secondhydraulic type speed change unit 20 in the primary speed changemechanism 1′, the graph concurrently showing speed changes between afirst speed position and a second speed position;

FIG. 18 shows time-transitional graphs, which are similar to the above,regarding speed changes between the second speed position and a thirdspeed position;

FIG. 19 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shifting-up time in arated-speed engine revolution state;

FIG. 20 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shift-down time in therated-speed engine revolution state;

FIG. 21 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shifting-up time in alow-speed engine revolution state;

FIG. 22 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shift-down time in a low-speedengine revolution state;

FIG. 23 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shifting-up time in therated-speed engine revolution state in a case where a delay time is notchanged depending whether one or two disengagement/engagement clutchesare operated;

FIG. 24 shows time-transitional graphs, which are similar to the above,regarding hydraulic-clutch-operating pressures at a shift-down time;

FIG. 25 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shifting-up time in therated-speed engine revolution state in a case where ahydraulic-pressure-increase start time of an engagement-objective clutchis controlled to match a hydraulic-reduction start time of adisengagement-objective clutch;

FIG. 26 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shifting-up time in therated-speed engine revolution state in a case where a delay time setbetween the pressure-reduction start time of the disengagement-objectiveclutch and the pressure objective start of the engagement-objectiveclutch is controlled shorter than the delay time set in the case shownin FIG. 19;

FIG. 27 is a flowchart for detection of an abnormal clutch and forhydraulic-pressure control of the hydraulic clutch according thereto;and

FIG. 28 shows diagram of other hydraulic circuits in the primary speedchange mechanism 1.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a transmission system used for a work vehicle (tractor)that has a nine-step-variable-speed-type speed change mechanism (aso-called hydraulic power shift speed change mechanism) includinghydraulic clutches as a primary speed change mechanism. The transmissionsystem is configured such that a travel transmission system and a PTOtransmission system are separated from an engine shaft 12 connectedthrough a buffering coupler 11 to an engine 10 disposed on a foremostportion of the vehicle as shown in FIG. 7. The transmission system ishoused in a transmission housing 2 shown in FIG. 7.

Hereinbelow, the transmission system shown in FIG. 1 will be described;and first, the travel transmission system will be described. A hydraulicreverser unit 14 is disposed between a reverser output shaft 13, whichis disposed parallel to the engine shaft 12, and the engine shaft 12. Afirst drive shaft 15 is disposed along an extending line of the reverseroutput shaft 13, and is integrally connected to the reverser outputshaft 13. A tube-type first speed change shaft 16 is disposed on anextending line of the engine shaft 12. A first hydraulic type speedchange unit 17 is provided between the first drive shaft 15 and a firstspeed change shaft 16. A tube-type second drive shaft 18 is disposedalong an extending line of the first speed change shaft 16. A secondspeed change shaft 19 is disposed along an extending line, of the firstdrive shaft 15. A second hydraulic type speed change unit 20 is providedbetween the second drive shaft 18 and the second speed change shaft 19.A propeller shaft 22 is disposed on extending line of the second speedchange shaft 19. A mechanical speed change mechanism 23 is disposed as asecondary speed change mechanism between the second speed change shaft19 and the propeller shaft 22. A small bevel gear 24 engages a largeinput bevel gear 26 of a left/right rear-wheel differential mechanism25. A differential output shaft 27 on the left/right of the differentialmechanism 25 is connected to a left/right rear wheel 30 shown in FIG. 8through a left/right brake 28 and final speed-reduction device 29 of aplanetary gear type. A diff-lock clutch 31 is provided on one of thedifferential output shafts 27.

In the travel transmission system, the primary speed change mechanism 1is constituted by combining the first hydraulic type speed change unit17 and the second hydraulic type speed change unit 20. However, beforeit is described, the hydraulic reverser unit 14 and the speed changemechanism 23 will hereinbelow be described in detail.

In the hydraulic reverser unit 14, a forward gear train 91 and abackward gear train 92 including an idle gear 92 a are provided betweenthe engine shaft 12 and the reverser output shaft 13. In the individualgear trains 91 and 92, gears are disposed to be idle on the engine shaft12. One of these gears on the engine shaft 12 is connected to the engineshaft 12 by alternative connection through one of a forward hydraulicclutch 14F and a backward hydraulic clutch 14R. Thereby, the forward orbackward rotation is selectively transmitted to the reverser outputshaft 13.

The speed change mechanism 23 allows a countershaft 21 to be connectedto the second speed change shaft 19 via a reduction gear train. Twospeed change gears 93 and 94 are immobilized on the countershaft 21. Viaa reduction gear mechanism 95, the speed change gear 94 on a smallerdiameter side of the shaft is connected to a speed change gear 96disposed outside of the countershaft 21. On the other hand, on thepropeller shaft 22, gears 97, 98, and 99 are provided to be idle, and inaddition, two dual-type clutches 100 function to selectively connect oneof the gears 98 and 99 to the propeller shaft 22. The dual-type clutch101 provides one of two selectable connections, one of the connectionsconnects the gear 97 to the propeller shaft 22, and the other connectiondirectly connects the second speed change shaft 19 and the propellershaft 22 together. As described above, four-step speed change can beimplemented according to the mechanical speed change mechanism thatfunctions as the secondary speed change mechanism.

The aforementioned tractor can travel either by two-wheel driving of theleft and right rear wheels 30 to which power is transmitted through thetravel transmission system or by four-wheel driving in which left andright front wheels 6 shown in FIG. 8 are also selectively driven. In atransmission system for the front-wheel driving power, two gears 34 and35 are idly provided on a countershaft 33 to be integrally andrelatively rotatable thereon. A gear 32 immobilized on the propellershaft 22 engages the gear 34; and gears 36 and 37 engage the gears 34and 35, respectively. Between the gears 36 and 37 and a driving-powertaking off shaft 38, a hydraulic clutch unit 39 is provided toselectively connect one of the gears 36 and 37 to thedriving-power-taking-off shaft 38. When the gear 37 is connected to thedriving-power-taking-off shaft 38, the front wheels 6 and the rear wheel30 rotate at a synchronizing speed. Also, when the gear 36 is connectedthereto, the front wheels 6 rotate at a speed higher that of the rearwheels 30.

Hereinbelow, the PTO transmission system will be described. Atransmission shaft 40 extends from the rear end of the engine shaft 12,passing through the tubular-type first speed change shaft 16, seconddrive shaft 18, and countershaft 21. A transmission shaft 41 extendsfrom the rear end of the transmission shaft 40. A PTO clutch 42 isprovided between the transmission shaft 41 and a transmission shaft 43that is provided on an extending line of the transmission shaft 41. APTO shaft 44 is disposed parallel to the transmission shaft 43 to extendoutside of the mechanism. Inside the mechanism, a mechanical PTO speedchange device 45 is provided between the transmission shaft 43 and thePTO shaft 44. Through gears 46, 47, and 48, the transmission shaft 41transmits power to a power-taking-off shaft 49 to drive a hydraulic pump50. The hydraulic pump 50 discharges pressurized fluid is used tooperate hydraulic clutches of the first hydraulic type speed change unit17 and the second hydraulic type speed change unit 20. In this case,fluid discharging from the hydraulic pump 50 may be used to verticallymove a hydraulic work-machine lifting device provided in a rear portionof the tractor.

Hereinbelow, the primary speed change mechanism 1 in the traveltransmission system will be described in detail. In the first hydraulictype speed change unit 17, three gears 51, 52, and 53 are provided to beidle on the first drive shaft 15, and respectively engage three gears54, 55, and 56 immobilized on the first speed change shaft 16. Therespective gears 51, 52, and 53 are alternatively connected to the firstdrive shaft 15 through three hydraulic clutches 57, 58, and 59 providedon the first drive shaft 15 to thereby allow three-step speed changes tobe implemented.

In the first hydraulic type speed change unit 20, three gears 60, 61,and 62 are provided to be idle on the first drive shaft 18, andrespectively engage three gears 63, 64, and 65 immobilized on the firstspeed change shaft 19. The respective gears 63, 54, and 65 arealternatively connected to the first drive shaft 19 through threehydraulic clutches 66, 67, and 68 provided on the first drive shaft 19to thereby allow three-step speed changes to be implemented.

The primary speed change mechanism 1 is configured to include the firsthydraulic type speed change unit 17 and the second hydraulic type speedchange unit 20 that are connected together in tandem. When one of thehydraulic clutches 57, 58, and 59 is connected to one of the hydraulicclutches 66, 67, and 68, totally, nine-step speed changes can beobtained.

As shown in Table 1, according to combinations of alternativeconnections of the hydraulic clutches 57, 58, and 59 and alternativeconnections of the hydraulic clutches 66, 67, and 68, the first andsecond hydraulic type speed change units 17 and 20 are set so as toobtain first to ninth speed ratios (output rotation speed/input rotationspeed; i.e., the rotation speed of the second speed change shaft19/rotation speed of the first drive shaft 15)

TABLE 1 Hydraulic clutches Hydraulic clutches connected in the connectedin the first hydraulic type second hydraulic type Speed ratios speedchange unit 17 speed change unit 20 1st 57 66 2nd 58 66 3rd 59 66 4th 5767 5th 58 67 6th 59 67 7th 57 68 8th 58 68 9th 59 68

Hereinbelow, a description will be made regarding a primaryspeed-changing hydraulic circuit shown in FIG. 2. The circuit isprovided for operating the hydraulic clutches 57, 58, and 59 in thefirst hydraulic type speed change unit 17, and the hydraulic clutches66, 67, and 68 in the second hydraulic type speed change unit 20. Thehydraulic pump 50 shown in FIG. 1 discharges fluid having a hydraulicpressure set by a pressure-controller valve 69 to a fluid-feeder circuit70.

The fluid-feeder circuit 70 is separated to branch circuits that areconnected to the aforementioned six hydraulic clutches 57, 58, 59, 66,67, and 68. In the individual branch circuits, two-position-methodelectromagnetic proportion selector valves VL, VM, VH, V1, V2, and V3are provided. For the convenience of description, a variable aperture Vaformed in each of the electromagnetic proportion selector valves isshown outside of each of the electromagnetic proportion selector valves.

Solenoids SL, SM, SH, S1, S2, and S3 of the respective electromagneticproportion selector valves VL, VM, VH, V1, V2, and V3 are eachcontrolled to an operational position through excitation. They are eachcontrolled to a neutral position when nonexcited. That is, when each ofthe solenoids is excited, a hydraulic clutch corresponding thereto iscontrolled to engage; whereas, when it is relieved from excitation, ahydraulic clutch corresponding thereto is controlled to disengage.

Pressure sensors PSL, PSM, PSH, PS1, PS2, and PS3 are connected,respectively, between the electromagnetic proportion selector valves VL,VM, VH, V1, V2, and V3 in the respective branch circuits and thehydraulic clutches 57, 58, 59, 66, 67, and 68. These pressure sensorsdetecting operating hydraulic pressures may be each connected to aswitch that performs ON/OFF operations relative to a predeterminedpressure value. Alternatively, the pressure sensor itself may beconstructed as the above sensor. Pressure sensors shown in FIGS. 17, 18,and 27 are individually configured as switches that turn ON when apressure value equal to or greater than a predetermined pressure value(switching pressure pb described below) and that turn OFF when thepressure value is lower than the value. However, they may be configuredto turn OFF when the pressure value is lower than the predeterminedpressure value and to turn ON when the pressure value is higher than thevalue.

The above-described electromagnetic proportion selector valves andpressure sensors are all stored in the primary-speed-change hydraulicvalve unit 3, as shown in FIG. 9. The valve unit 3 is disposed in a partof the tractor, as shown in FIG. 8, and a the valves and sensors areconnected to hydraulic-clutches in the transmission housing 2 throughpipings. FIG. 9 shows the primary-speed-change hydraulic valve unit 3that stores electromagnetic proportion selector valves and pressuresensors of the primary speed change mechanism 11, which will bedescribed below. While the primary speed change mechanism 1′ will bedescribed below in detail, it is briefed hereinbelow. It is configuredby tandem connection of the first hydraulic type speed change unit 17′and the second hydraulic type speed change unit 20. The first hydraulictype speed change unit 17′ is configured by eliminating anintermediate-speed clutch 58 from the first hydraulic type speed changeunit 17 in the nine-step primary speed change mechanism 1. The secondhydraulic type speed change unit 20 is the same as that in the primaryspeed change mechanism 1. Because of this configuration, the train ofthe above-described electromagnetic proportion selector valves and thetrain of the above-described pressure sensors are stored in theprimary-speed-change hydraulic valve unit 3 in a state where theelectromagnetic proportion selector valve VM (and the solenoid SM) andthe pressure sensor PSM are removed.

A description will hereinbelow be made referring to back to thehydraulic circuit diagram in FIG. 2. A lubricant-pressure-settingsecondary pressure controller valve 72 is connected to a drain side of apressure controller valve 69 that branches from the fluid-feeder circuit70. A lubricant circuit 73 is led from a portion between the twopressure controller valves 69 and 72 to feed lubricant to the hydraulicclutches 57, 58, 59, 66, 67, and 68.

A line filter 76 and a relief valve 77 that functions as a bypass valveare parallel-connected to a fluid-drawing-in circuit 75 that extendsfrom a fluid reservoir 74 up to the hydraulic pump 50. When the linefilter 76 is incidentally blinded, the relief valve 77 performs a reliefoperation to maintain lubricant to flow to the hydraulic pump 50.

A hydraulic pump 78 driven by the engine shaft 12, as shown in FIG. 1(not shown in FIG. 2), discharges fluid to the two hydraulic clutches14F and 14R of the above-described hydraulic reverser unit 14. Afluid-drawing-in circuit 79 is provided to connect a fluid-drawing-inside of the hydraulic pump 78 and the fluid-drawing-in circuit 75 toalso feed fluid in the fluid reservoir 74 to the hydraulic clutches 14Fand 14R.

Hereinbelow, referring to FIG. 3, a description will be made regardingelectrical operation control of the electromagnetic proportion selectorvalves VL, VM, VH, V1, V2, and V3 for the primary speed change mechanism1. An input-side interface of a logical circuit 80, as shown in FIG. 8,in a controller 4 disposed in part of the tractor is connected to apotentiometer 82, a tachometer 83, a mode-selector switch 84, and theabove-described six pressure sensors PSL, PSM, PSH, PS1, PS2, and PS3.The potentiometer 82 detects the position (lever angle) and therotational direction of a primary speed change lever 81 disposed near anoperator seat 7, as shown in FIGS. 7 and 8. The speeds are set in therange of first to ninth speed positions in the order from lower tohigher speeds; and as shown in FIG. 3, numbers 1 to 9 corresponding tothe speed positions are indicated in a rotational area of the primaryspeed change lever 81. The tachometer 83 detects the revolutionfrequency of the engine 10. The mode-selector switch 84 functions inresponse to an operation performed by an operator to vary ahydraulic-pressure-increase property that causes the hydraulic clutchesin the primary speed change device to engage corresponding to a normal(on-the-road) travel mode and a work traveling mode that is to becarried out at a tractional load. An electronic governor may also beused as a selector switch between two control modes. One of the controlmodes is carried out to adjust the frequency of the engine revolution toa revolution frequency at the time of normal acceleration operation. Theother control mode is carried out to detect engine load ratios, andcontrols the engine revolution frequency corresponding to the loadratios.

However, incidents can occur in which an operator overlooks operationsof the mode-selector switch 84, the traction load actually exerted isnot greater than a logical load even when the above-described switch isset to the work traveling mode, thus causing ahydraulic-pressure-increase property that is different from an actualproperty. Taking the above into account, two embodiments of thehydraulic pressure control are disclosed below with reference to FIGS.12 and 13 and FIGS. 14 to 16. The individual embodiment include anautomatic load detection structure that is capable of makingself-determination as to which one of the control modes should beselected to allow the hydraulic-pressure-increase property for thehydraulic clutches to be selected corresponding to actual states. Inconjunction with these control modes, there are provided left and rightdraft sensors 122L and 122R L to right and left lower links 121 in awork-machine-attaching device 120, a traction sensor 123 on a draw-bar(not shown), and an electronic governor controller 5. FIG. 4 disclosesan electrical controller circuit. In the circuit, instead of themode-selector switch 84, the left and right draft sensors 122L and 122R,the traction sensor 123, and the electronic governor controller 5, whichare input means for hydraulic-clutch pressure-increase-propertyselection, are connected to the input-side interface of the logicalcircuit 80. A tachometer 83 and a rack-position sensor 124 are connectedto the input side of the logical circuit 80. Through the electronicgovernor controller 5, the logical circuit 80 receives a signal inputfrom the tachometer 83, and in addition, a load-ratio signal obtainedthrough calculation performed according to signals input from thetachometer 83 and the rack-position sensor 124. In addition, theelectronic governor controller 5 is connected to a hydraulic liftcontroller 125 for hydraulically driving the work-machine-attachingdevice 120 and to an electronic governor 126 (a driving device for afuel-injection-amount controller rack).

In each of the electrical controller circuits shown in FIGS. 3 and 4, anoutput side of the logical circuit 80 is connected to a delay circuit 88that is connected to the input side of a solenoid-driver circuit 86 andto a solenoid-driver circuit 85. The solenoid-driver circuit 85 drivessolenoids SL, SM, SH, S1, S2, and S3 of the electromagnetic proportionselector valves VL, VM, VH, V1, V2, and V3 in an exiting direction. Thesolenoid-driver circuit 86 drives these solenoids to be relived fromexcitation.

The output side of the logical circuit 80 is connected to thesolenoid-driver circuit 85 and a pressure (—increase-property)—settingcircuit 87. The output side of the pressure-setting circuit 87 isconnected to the solenoid-driver circuit 85. The pressure-settingcircuit 87 stores two types of solenoid excitation patterns that areused to obtain two types of pressure-increase properties as representedby pressure-increase graphs U1 and U2 shown in FIG. 10.

In addition, the output side of the logical circuit 80 is connected tothe delay circuit 88, a time-setting circuit 90, and a pressure(—decrease-property)—setting circuit 89. The output side of thetime-setting circuit 90 is connected to the delay circuit 88. Thesolenoid-driver circuit 86 is connected to the output side of the delaycircuit 88 and to the output side of the pressure-setting circuit 89,and to the solenoid-driver circuit 86. The pressure-setting circuit 89stores three types of solenoid-excitation-relieving patterns that areused to obtain three types of pressure-decrease properties asrepresented by pressure-decrease graphs D1, D2, and D3 shown in FIG. 11.

In the logical circuit 80, an engagement-objective clutch anddisengagement-objective clutch are determined according to a signal thatrepresents postshift position of the primary speed change lever 81,which is detected through the potentiometer 82. In addition, accordingto a logic described below, a hydraulic-pressure-decrease property forthe disengagement-objective clutches are determined. The electricalcontroller circuit shown in FIG. 3 performs setting through themode-selector switch 84. On the other hand, the electrical controllercircuit shown in FIG. 4 inputs signals from the right and left draftsensors 122 and the traction sensor 123. The circuit determines apressure-increase property for engagement-objective clutches accordingto inputs from the electronic governor controller 5. In addition,according to inputs from the pressure sensors PSL, PSM, PSH, PS1, PS2,the circuit determines the necessity for control that is performed toprevent entrance of foreign substances to the hydraulic clutches.

The logical circuit 80 sends a signal to the solenoid-driver circuit 85.This signal causes the solenoid-driver circuit 85 to send an ON-signalto a solenoid for an objective electromagnetic proportion selectorvalve. Concurrently, the logical circuit 80 sends to thepressure-setting circuit 87 a pressure-setting signal for selecting oneof the solenoid excitation patterns. Thereby, control is performed fortransmission of the ON-signal to the solenoid according to the solenoidexcitation pattern that has been set in the pressure-setting circuit 87.

Similarly, the logical circuit 80 sends a signal to the solenoid-drivercircuit 86, and the signal causes the solenoid-driver circuit 86 to sendan OFF-signal to a solenoid for an objective electromagnetic proportionselector valve. Concurrently, the logical circuit 80 sends to thepressure-setting circuit 89 a pressure-setting signal for selecting oneof the solenoid nonexcitation patterns. Thereby, control is performedfor transmission of the OFF-signal to the solenoid according to thesolenoid excitation pattern that has been set in the pressure-settingcircuit 89.

In addition to the logical circuit 80, similar electrical controllercircuits 85, 86, 87, 88, 89, and 90 are provided either in theabove-described controller 4 or in the primary-speed-change hydraulicvalve unit 3. According to the solenoid-driver circuit 85 and thesolenoid-driver circuit 86, control signals (ON/OFF signals) are sent toobjectives of the solenoids SL, SM, SH, S1, S2, and S3 of theelectromagnetic proportion selector valves VL, VM, VH, V1, V2, which areprovided in the primary-speed-change hydraulic valve unit 3.

Hereinbelow, a description will be made regarding the transmissionsystem for the work vehicle (tractor) equipped with the primary speedchange mechanism 1′ of the six-step-speed-change type, which is shown inFIG. 5.

Individual components and constructions in the transmission system arethe same as those shown in FIG. 1, except for the first hydraulic typespeed change unit 17′. The first hydraulic type speed change unit 17′shown in FIG. 1 is configured to exclude the intermediate-speed-stepgear train, that is, the gears 52 and 55, to thereby enable two-stepspeed changes. In addition, with the overall primary speed changemechanism 1′ configured to include the combination of the firsthydraulic type speed change unit 17′ and the second hydraulic type speedchange unit 20, totally six-step speed changes are enabled.

Specifically, as shown in FIG. 2, according to combinations ofalternative connections of the hydraulic clutches 57 and 59 andalternative connections of the hydraulic clutches 66, 67, and 68, thefirst and second hydraulic type speed change units 17′ and 20 are set soas to obtain first to sixth speed ratios (output rotation speed/inputrotation speed; i.e., the rotation speed of the second speed changeshaft 19/rotation speed of the first drive shaft 15)

TABLE 2 Hydraulic clutches Hydraulic clutches connected in the firstconnected in the hydraulic type speed second hydraulic type Speed ratioschange unit 17 speed change unit 20 1st 57 66 2nd 59 66 3rd 57 67 4th 5967 5th 57 68 6th 59 68

FIG. 6 shows a hydraulic-clutch-controlling hydraulic circuit in theprimary speed-changing mechanism 1′ shown in FIG. 5. The same referencenumerals/symbols as those shown in FIG. 2 represent the same membersshown therein. Although an electrical controller circuit is notdisclosed therein, it is configured such that the pressure sensor PS andthe solenoid SM are removed from the electrical controller circuit shownin FIG. 4 or 5, and first to sixth speed positions of a primary speedchange lever 81 therein are included.

FIGS. 7 and 8 each show the tractor employing the electrical controllersystem shown in either FIG. 1 or FIG. 5. The members shown with thereference numerals have already been described in connection with thetransmission system shown in FIGS. 1 to 4. The tractor includes aload-detecting means used in determination of a pressure increaseproperty for engagement-objective clutches, and employs the electricalcontroller circuit shown in FIG. 4 rather than that shown in FIG. 3. Inaddition, the primary-speed-change hydraulic valve unit 3 shown in FIG.9 is disposed in the position shown in FIG. 8, and as described above,it is intended for the six-step-type primary speed change mechanism 11shown in FIG. 5. To use it for the nine-step-type primary speed changemechanism 1 shown in FIG. 1, the configuration may be modified such thatthe valve device is replaced with a valve device in which theelectromagnetic proportion selector valve VM and the pressure sensor PSMare added and stored.

Hereinbelow, a description will be made regarding the hydraulic pressurecontrol in the hydraulic-clutch-included speed change mechanism of thepresent invention. The hydraulic pressure control described below may beapplied either to the nine-step-type primary speed change mechanism 1shown in FIG. 1 or to the six-step-type primary speed change mechanism1′ shown in FIG. 5.

FIG. 10 shows a pressure-increase property for an engagement-objectiveclutch at the time of speed-changing. Specifically, from an engagementstart time t₀ when an ON-signal is fed to an objective solenoid(excitation is started), a clutch-operating hydraulic pressure p isgradually increased to finally reach a normal hydraulic pressure p₁, asshown by pressure-increase graphs U1 and U2.

In the pressure-increase graphs U1 and U2, the low-levelpressure-increase graph U1 is set at a road-travel time when a travelload is low, whereas the high level pressure-increase graph U2 is set ata work-travel time when the travel load is high. When the travel load ishigh, torque transmission efficiency needs to urged to increase, and aload resistance force needs to be exerted. At the road-travel time whenthe travel load is low, since amenity is required, shock that can occuraccording to hydraulic-pressure rise at the time of clutch-shiftoperation needs to be minimized. In a configuration using the electricalcontroller circuit shown in FIG. 3, an operator uses the mode-selectorswitch 84 to determine which one of the pressure-increase graphs U1 andU2 is set. In a configuration using the electrical controller circuitshown in FIG. 4, determination regarding which one of thepressure-increase graphs U1 and U2 is set is dependent on determinationthat is made in the logical circuit 80. The determination is made in thelogical circuit 80 according to either signals input from the right/leftdraft sensors 122 and the traction sensor 123 shown in FIG. 8 or anengine-load-ratio signal that is input through the electronic governorcontroller 5.

Hereinbelow, referring to FIGS. 12 and 13, a description will be maderegarding a method for determining a pressure-increase property for anengagement-objective clutch according to load detection that isperformed using the right and left draft sensors 122L and 122R and thetraction sensor 123.

When the right/left lower link 121 is pulled backward, the right/leftdraft sensor 122L/122R detects a load thereof. Depending on the pullingforce, variations occur in the value of voltage input to the logicalcircuit 80. In FIG. 12, the sum of output voltage values of the twodraft sensors 122 is represented by a load voltage value L. When theload voltage value L is a value L1 at a normal (on-the-road) traveltime, the load voltage value L is a value L2, which is lower than thevalue L1, in a tractional-work travel time. A threshold value L3 is setbetween the values L1 and L2, and it is assumed that a case where theload voltage value L is equal to or lower than the threshold value L3 isrepresented as a selection zone of a pressure-increase property for aprimary-speed-change hydraulic clutch at a traction-load mode.

On the other hand, the traction sensor 123 turns OFF at a normal(on-the-road) travel time, and it turns ON upon being imposed by a loadat a tractional-work travel time. When the traction sensor 123 is turnedON, selection is made for a pressure-increase property for theprimary-speed-change hydraulic clutch at a tractional-load mode.

Specifically, as shown in a flowchart in FIG. 13, in at least one of thecases where the load voltage value L input from the right/left draftsensor 122 to the logical circuit 80 is equal to or less than L3 (step201) and where the traction sensor 123 is turned ON (step 202), thetractional-load-mode pressure-increase graph U2 is selected (step 204).In the other case, i.e., when the load voltage value L of the right/leftdraft sensor 122L/122R is turned OFF, and concurrently, the tractionsensor 123 is turned OFF, the normal-travel-mode pressure-increase graphU1 is selected (step 203).

Hereinbelow, referring to FIGS. 14 to 16, an introduction is maderegarding another embodiment that determines a pressure-increase patternaccording to detection by the electronic governor of the engine forengine loads (load ratios). The embodiment of the control may beemployed by a vehicle mounting a diesel engine corresponding to theengine 10.

As shown in FIG. 4, the electronic governor controller 5 is connected tothe input-side interface of the logical circuit 80 to thereby allowload-ratio signals in the electronic governor controller to be input tothe logical circuit 80. The electronic governor controller 5 isconnected to a hydraulic-pressure lift controller 125, theabove-described tachometer 83 that detects the engine revolutionfrequency, and the rack-position sensor 124 that detects the position ofthe fuel-injection-amount controller rack of the governor.

The electronic governor controller 5 calculates load ratios according toinputs from the tachometer 83 and the rack-position sensor 124. Inaddition, it inputs an engine-load-ratio signal, which is output basedon the load ratios, to the hydraulic-pressure lift controller 125 tothereby use engine-load-ratio signal to lift a hydraulic lift of thework-machine-attaching device 120. Concurrently, the electronic governorcontroller 5 inputs an engine-load-ratio signal, uses an output controlsignal fed back from the logical circuit 80 to move the rack, andthereby controls the fuel-injection amount. Among theseengine-load-ratio signals issued from the electronic governor controller5, the signal for the hydraulic-pressure lift controller 125 is outputat a long frequency to prevent overcontrol that can reduce workefficiency. The signal for the logical circuit 80 is output at a shortfrequency so that the engine revolution frequency can be quicklycontrolled corresponding to the load. By use of the signal output to thelogical circuit 80 at the short frequency without performingmodification, pressure-increase properties can be quickly determinedcorresponding to load ratios, thereby allowing the signal to beeffectively used for hydraulic-pressure control of the hydraulic clutch.

As shown in FIG. 14, an engine revolution frequency Ne represented by avoltage input from the tachometer 83 to the electronic governorcontroller 5 is assumed to be changed from Ne3 to Ne1 that is lower thanNe3. On the other hand, as shown in FIG. 15, a rack position Ls, ofwhich data has been input from the rack-position sensor 124 to theelectronic governor controller 5, is assumed to be changed from Ls3 toLs1 (on a side where the fuel-injection amount is relatively large) thatis higher than Ls3. In this way, when the reduction in the enginerevolution frequency and the lifting-up (i.e., increase in thefuel-injection amount) concurrently occur, and in addition, theindividual reduction and lifting-up appear with specific properties,determination is made such that a tractional load is imposed on thevehicle, and the pressure-increase graph U2 is set in thepressure-setting circuit 87.

Hereinbelow, referring to FIGS. 16A and 16B, a description will be maderegarding a pressure-increase-property setting flow that is carried outbased on detection of the engine revolution frequency and the rackposition. First, as prerequisite processing, the tachometer 83 detectsengine revolution frequencies Ne at a specific short frequency, therack-position sensor 124 detects rack positions Ls at the same frequencyas that for the engine revolution frequencies Ne, and values of thedetections are serially stored therein. In step 301, among enginerevolution frequencies Ne serially detected, the circuit stores at leastengine revolution frequencies Ne2 (immediate-previously detectedrevolution frequency), Ne3 (second-previously detected revolutionfrequency), Ne4 (third-previously detected revolution frequency), andNe5 (fourth-previously detected revolution frequency). Concurrently (instep 312 in the flow for the convenience of description), among rackpositions Ls serially detected, the system stores at least rackpositions Ls2 (immediate-previously detected position at t₂), Ls3(second-previously detected position at t₃), Ls4 (third-previouslydetected position at t₄), and Ls5 (fourth-previously detected positionat t₅).

Then, a new engine revolution frequency Ne is detected by the tachometer83 at a current detection start time t₁, and a signal representing acurrently detected engine revolution frequency Ne1 is input to theelectronic governor controller 5 (step 302). Then, the engine revolutionfrequency Ne2 immediate-previously detected is retrieved, and thecurrent engine revolution frequency Ne1 is compared to the previous Ne2to verify Whether the reduction in the intended engine revolutionfrequency has been achieved; that is, it verifies whether Ne1<Ne2 hasbeen achieved (step 303). After the verification of the reduction in theengine revolution frequency, a calculation of a reduction amount al(=Ne2−Ne1) is performed (step 304). In addition, data of the storedengine revolution frequencies Ne2, Ne3, Ne4, and Ne5 is retrieved, andverification is performed for at least the reduction in the enginerevolution frequency from the fourth-previously detected frequency.Subsequently, calculations are performed to obtain reduction amounts a2(=Ne3−Ne2), a3 (=Ne4−Ne3), and a4 (=Ne5−Ne4) (steps 305 to 310) tothereby verify whether a1−a2>a3−a4, that is, the increase in thereduction ratio of the engine revolution frequency, has been achieved(step 11).

If the engine revolution frequency is reduced, and the reduction amountper unit time in that case is increased, it is conceivable that thespeed has been reduced because of either acceleration setting or atractional load. If the speed has been reduced because of theacceleration setting, the rack position in an electronic governor 126 issupposed to be at a fuel-injection-amount reduced side (a rack-positiondetection voltage should have been reduced). On the other hand, in acase where the engine revolution frequency has been reduced, but therack position has been shifted to a fuel-injection-amount increased side(the rack-position detection voltage is increased) despite of the factthat the engine revolution frequency has been reduced, the case can bedetermined that the electronic governor 126 has performed controlcorresponding to the load.

Under the above concepts, the Ls2, Ls3, Ls4, and Ls5 are stored (step312), as described above. In-this state, a new rack position Ls isdetected by the rack-position sensor 124 at a current detection starttime t₁, and a signal representing a detected rack position Ls1 is inputto the electronic governor controller 5 (step 313). Then, data of theengine revolution frequency Ls2 immediate-previously detected isretrieved, and the current rack position Ls1 is compared to the previousLs2 to thereby verify whether the rack position has been lifted up(shifted to a fuel-injection-amount increased side), that is, to verifywhether Ls1>Ls2 has been achieved (step 314). After the verification ofthe lift-up in the rack position, a calculation of a reduction amount b1(=Ls1−Ls2) is performed (step 315). In addition, stored data of the rackpositions Ls2, Ls3, Ls4, and Ls5 is retrieved, and verification isperformed for at least the reduction in the engine revolution frequencyfrom the fourth-previously detected result. Subsequently, calculationsare performed to obtain reduction amounts b2 (=Ls3−Ls2), b3 (=Ls4 Ls3),and b4 (=Ls5−Ls4) (steps 316 to 321) to thereby verify whetherb1−b2>b3−b4, that is, the increase in the increase-shift ratio regardingthe rack position, is achieved (step 322).

As described below, the pressure-decrease properties (pressure-decreasegraphs) are determined such that the solenoid nonexcitation patterns inthe pressure-setting circuit 89 are selected corresponding to enginerevolution frequencies and the like detected by the tachometer 83. Inorder to allow the pressure-increase property to be modifiedcorresponding to loads, the pressure-decrease property fordisengagement-objective clutches may also be established to be modifiedbased on engine-load-ratio signals input to the logical circuit 80 fromeither load-ratio-detecting means such as the right and left draftsensors 122 and the traction sensor 123 or the electronic governorcontroller 5.

Hereinbelow, referring back to FIG. 10, a detailed description will bemade regarding the overall increase processing of an operating hydraulicpressure p. A solenoid for an objective hydraulic clutch at aclutch-engagement start time t₀ is turned ON, and supply of fluid to afluid chamber of the objective hydraulic clutch is started. Theoperating hydraulic pressure p in the fluid chamber slightly rises atthe time t₀, and then gradually increases. At a time ta when some timehas passed from the time t₀, the fluid chamber becomes full of fluid,upon detection of arrival of hydraulic pressure p at a piston-holdingpressure P_(a) (i.e., a pressure allowing a piston to operate), thehydraulic pressure p is quickly increased to a normal pressure at thattime.

In a period up to a time tb, as shown in part of each of thepressure-increase graphs U1 and U2, the hydraulic pressure p graduallyincreases, and the clutch stays at a slip state. At the time tb, thehydraulic pressure p reaches a value required for complete engagement ofthe clutch. Subsequently, as shown in a part b of each of thepressure-increase graphs U1 and U2, the hydraulic pressure p isgradually increased up to a normal pressure P₁ to thereby cause theclutch to a pressed state. When the hydraulic pressure p reaches thenormal pressure P₁, the clutch engagement is completed.

FIG. 11 shows a hydraulic-pressure-decrease property for adisengagement-objective clutch at the time of speed-changing.Specifically, it shows cases where, from a time ts when an OFF-signal isapplied to an objective solenoid (excitation is started therefor), aclutch-operating normal pressure p, is reduced as represented byhydraulic-pressure-decrease property graphs D1, D2, and D3. Thehydraulic-pressure-decrease property graph D1 represents a case where,immediately after the solenoid is relieved from excitation, the pressurep is quickly reduced from the normal pressure p₁ to a piston-holdingpressure P_(a). The hydraulic-pressure-decrease property graphs D2 andD3 each represent a case where after the pressure p is quickly reducedto a pressure that is higher than the piston-holding pressure P_(a), itis gradually increased to 0 (or the lowest value in the vicinity).Therefore, the reduction degree of D2 is greater than that of D3.

In either case, the hydraulic clutch is urged toward a neutral position.The overall disengagement period of the clutch, i.g., a period in whichthe pressure p is changed to 0 (or the lowest value in thevicinity)(even in the case of setting of the hydraulic-pressure-decreaseproperty graph D3 representing the slowest pressure reduction) isshorter than the overall disengagement period described above.

Pressure-decrease property graphs are not limited to the three graphs D1to D3. The angle in the gentle-sloped pressure-decrease portion as canbe seen in either D2 or D3 may be variously set so that control asrepresented by other pressure-decrease property graphs can beimplemented. However, for the convenience of description, embodimentsshown in FIGS. 19 to 22, which will be described below, are assumed tohave the capacity of performing control represented by thehydraulic-pressure-decrease property graphs D1, D2, and D3.

The pressure is gradually increased, and the hydraulic-pressure-increaseproperty is varied in the course from a to b according to the control ofan application voltage to each of the solenoids. In addition, thevariable aperture of the individual electromagnetic proportion selectorvalve is used to slowly vary the hydraulic pressure as represented bythe hydraulic-pressure-decrease property graph D2 or D3. In thehydraulic circuit diagram shown in FIG. 2, the individual variableaperture is shown in outside portion with reference symbol Va. Accordingto the variations in the voltage applied to the individual variableaperture Va, the amount of drain from the individual electromagneticproportion selector valve is controlled, thereby causing the decreasebehavior of the hydraulic pressure to vary.

Hereinbelow, referring to FIGS. 17 to 26, a description will be maderegarding the relationship between the engagement course and thedisengagement course of a clutch at time of speed-changing.

First, as a basic concept, either the first hydraulic type speed changeunit 17 or the second hydraulic type speed change unit 20 is configuredso as not to encounter total power cut during speed-changing. When oneof the speed change units encounters a non-transmissible state, that is,when all hydraulic clutches in one of the speed change units are held indisengaged states, transmission is not performed in the primary speedexchange device, that is, transmission is not performed between thefirst drive shaft 15 to the second speed change shaft 19. If work travelwas performed in the above state, the vehicle might be unexpectedlystopped, and in addition, a great shock giving discomfort would becaused by hydraulic-pressure rise according to clutch engagementperformed from the above state.

As described above, the clutch-engagement period is longer than theclutch-disengagement period (even with any one of the pressure-decreasepatterns being set), and the objective-clutch-operating hydraulicpressure is gradually increased. Taking the above into account, thepresent invention is arranged such that the reduction in hydraulicpressure of a disengagement-objective clutch is started during gradualincrease in pressure for an engagement-objective clutch. Thereby, aperiod in which a pressure p for operating the disengaging clutch iscontrolled to be higher than the piston-holding pressure P_(a) (a statewhere the clutch slips) is controlled to overlap a period in which apressure p for operating the engaging clutch is higher than thepiston-holding pressure P_(a) (a state where the clutch is slipping).That is, even when the transmission efficiency is reduced to thelowest-value level, either the disengagement-objective clutch or theengagement-objective clutch is controlled to slip, thereby avoiding acase where one of the clutches is forced to be in a disengaged state,and the primary speed change mechanism is forced to be in anon-transmissible state.

In this connection, for example, as shown in time-transitional graphs ofFIGS. 17 and 19 regarding hydraulic-clutch-operating hydraulic pressures(graphs each showing a hydraulic-clutch-operating pressure p relative toa time t), regions where an in-engagement clutch and an in-disengagementclutch commonly slip (hereinbelow, the aforementioned region will bereferred to as a “common slip region”) are shown by slanting lines. Thestate and the area of the common slip region are preferably set so thatspeed-changing (speed-position shifting) can be performed most smoothly;that is, good speed-change feeling can be secured without beinginfluenced by the capacity of the hydraulic pump 50.

To change the speed by shifting clutches of one of the primary speedchange mechanisms 1 and 1′ through shifting of the primary speed changelever 81, as can be seen from Tables 1 and 2, there are two cases. Inone of the cases, one hydraulic clutch is newly engaged, and anotherengaged hydraulic clutch is disengaged in only one of the firsthydraulic type speed change unit 17 (17′) and the second hydraulic typespeed change unit 20. In the other case, one hydraulic clutch is newlyengaged, and another engaged hydraulic clutch is disengaged in each ofthe two first hydraulic type speed change units 17 (17′) and 20.

In the former case, for example, the following operations are performed.In the primary speed change mechanism 1, when the primary speed changelever 81 is shifted up from the second speed position to the fifth speedposition, the engaged hydraulic clutch 58 is remained unchanged in thefirst hydraulic type speed change unit 17; and the hydraulic clutch 67is newly engaged, and the engaged hydraulic clutch 66 is disengaged inthe second hydraulic type speed change unit 20. When the primary speedchange lever 81 is shifted down from the sixth speed position to thefourth speed position, the engaged hydraulic clutch 67 is remainedunchanged in the second hydraulic type speed change unit 20; and thehydraulic clutch 57 is newly engaged, and the engaged hydraulic clutch59 is disengaged in the first hydraulic-type speed change unit 17.

In the latter case, for example, the following operations are performed.In the primary speed change mechanism 1, when the primary speed changelever 81 is shifted up from the second speed position to the sixth speedposition, the engaged hydraulic clutch 59 in the first hydraulic typespeed change unit 17 and the hydraulic clutch 67 in the second hydraulictype speed change unit 20 are newly engaged, and the hydraulic clutch 58in the first hydraulic type speed change unit 17 and the hydraulicclutch and hydraulic clutch 66 in the second hydraulic type speed changeunit 20 are disengaged. When the primary speed change lever 81 isshifted down from the ninth speed position to the fifth speed position,the engaged hydraulic clutch 58 in the first hydraulic type speed changeunit 17 and the hydraulic clutch 67 in the second hydraulic type speedchange unit 20 are newly engaged, and the hydraulic clutch 59 in thefirst hydraulic type speed change unit 17 and the hydraulic clutch andhydraulic clutch 68 in the second hydraulic type speed change unit 20are disengaged.

In the primary speed change mechanism 1′, in the former case, forexample, when the primary speed change lever 81 is either shifted up orshifted down between the first speed position and the second speedposition, the hydraulic clutch 66 is remained engaged in the secondhydraulic type speed change unit 20, one of the hydraulic clutches 57and 59 is engaged in the first hydraulic type speed change unit 17′, andthe other hydraulic clutch is disengaged. In the latter case, forexample, when the primary speed change lever 81 is either shifted up orshifted down between the second speed position and the third speedposition, engaged-clutch exchange is performed between the hydraulicclutches 57 and 59 in the first hydraulic type speed change unit 17, andengaged-clutch exchange is performed between the hydraulic clutches 66and 67 in the second hydraulic type speed change unit 20.

In short, two speed changes can be achieved. One of the speed changes isachieved such that, in the overall primary speed change mechanism, onehydraulic clutch is disengaged, and one hydraulic clutch is newlyengaged (which hereinbelow will be referred to as “speed-changing withone-objective-based hydraulic clutches being disengaged/engaged”). Theother speed change is achieved such that, in the overall primary speedchange mechanism, two hydraulic clutches are disengaged, and twohydraulic clutches are newly engaged (which hereinbelow will be referredto as “speed-changing with two-objective-based hydraulic clutches beingdisengaged/engaged”). In either case, it is essential to secure theaforementioned common slip region.

FIGS. 17 and 18 each show time-transitional graphs (graphs of operatingpressures p relative to a time t) regarding individualhydraulic-clutch-operating hydraulic pressures in the first hydraulictype speed change unit 171 and the second hydraulic type speed changeunit 20 on the same time axis. Concurrently, the figures each showtime-transitional voltage graphs regarding the pressure sensors. FIG. 17shows the speed change with one-objective-based hydraulic clutches beingdisengaged/engaged, in which the primary speed change lever 81 is eithershifted up and shifted down between the first speed position and thesecond speed position. FIG. 18 shows the speed-changing withtwo-objective-based hydraulic clutches being disengaged/engaged, inwhich the primary speed change lever 81 is shifted up and shifted downbetween the second speed position and the third speed position. For eachpressure-rising portion of the individual hydraulic-pressuretime-transitional graphs in FIGS. 17 and 18, thefluid-chamberfilling-out required period (engagement start time t, topressure-rising time ta) shown in FIG. 10 is not taken into account, andthe pressure is assumed to increase higher than the piston-holdingpressure pa as soon as the position of the primary speed change lever 81has been shifted. In addition, pressure sensors PSL, PSM, PSH, PS1, PS2,and PS3 shown in each of FIGS. 17 and 18 are each—assumed to function asa switch that turns ON when the pressure increases higher than aswitch-shifting pressure pb which is set higher than the piston-holdingpressure pa.

Hereinbelow, FIG. 17 will be explained. First, when the primary speedchange lever 81 is set either to the first speed position or to thesecond speed position, in the second hydraulic type speed change unit20, a hydraulic pressure 66 p is kept at the normal pressure P₁, thepressure sensor PS1 keeps turning ON, individual hydraulic pressures 67p and 68 p for the hydraulic clutches 67 and 68 remain to be 0, and thepressure sensors PS2 and PS3 are turned OFF.

In the first hydraulic type speed change unit 17′ when the primary speedchange lever 81 is shifted up from the second speed position to thefirst speed position, a hydraulic pressure 59 p for theengagement-objective clutch 59 begins to rise, and is then graduallyincreased to the normal pressure P₁. In this course, when a hydraulicpressure 59 p reaches the switch-shifting pressure ph, the pressuresensors PSH turns ON. Slightly later than the rise in the pressure 59 p,a hydraulic pressure 57 p for the disengagement-objective clutch 57begins to decrease, and a pressure-decrease line portion thereof crossesa pressureincrease line portion of the hydraulic pressure 59 p. That is,an operating hydraulic pressure for the disengagement-objective clutchdecreases in the course of gradual increase in an operating hydraulicpressure for the engagement-objective clutch. In this way, as shown bythe slanting lines, common slip regions of the two hydraulic clutches 57and 59 are secured. When the decreasing hydraulic pressure 57 p isreduced lower than the switch-shifting pressure pb, the pressure sensorPSL in the ON state turns OFF.

Then when the primary speed change lever 81 is shifted down from thesecond speed position to the first speed position, the hydraulicpressure 57 p for the hydraulic clutch 57 begins to rise, and thengradually increases: and the hydraulic pressure 59 p for the hydraulicclutch 59 decreases. As described above, common slip regions are securedas in the above case. In the pressure-rising course, when the hydraulicpressure 57 p reaches the switch-shifting pressure pb, the pressuresensor PSL in the OFF state turns ON. When the decreasing hydraulicpressure 59 p is reduced lower than the switch-shifting pressure pb, thepressure sensor PSH in the ON state turns off.

When the state at the shifted-up (from the first speed position to thesecond speed position) time is compared to the state at the time of theshifted-down (from the second speed position to the first speedposition), the common slip region at the shifted-down time is relativelynarrow. At the shifted-down time, since rotational inertia prior to theshifting (in the state of the second speed position) is imposed as atransmission force on a rotation shaft on the secondary side of theclutch, which is engaged/disengaged, the slip regions are controlled tobe narrow to allow the speed to be changed smoothly and quickly.

In FIG. 18 showing a case where the speed is changed between the secondspeed position and the third speed position, hydraulic clutches aredisengaged/engaged in each of the first hydraulic type speed change unit17′ and 20. Specifically, in the first hydraulic type speed change unit17′, hydraulic-pressure control as described referring to FIG. 17 isperformed for disengagement/engagement operation of the hydraulicclutches 57 and 59 individually at a time of shifting-up (from thesecond speed position to the third speed position) and at a time ofshifting-down (from the third speed position to the second speedposition). On the other hand, in the second hydraulic type speed changeunit 20, simultaneously with rise in the hydraulic pressure 57 p for thehydraulic clutch 57, the engagement-objective clutch 67 p begins torise, and then gradually increases parallel to the increase in thehydraulic pressure 57 p up to the normal pressure P₁. Simultaneously, inparallel to reduction in the hydraulic pressure 59 p for thedisengagement-objective clutch 59 in the first hydraulic type speedchange unit 17′, and slightly later than rise in the hydraulic pressure67 p, the hydraulic pressure 66 p for the disengagement-objective clutch66 decreases. At the shifting-down time, the hydraulic pressure 66 p forthe engagement-objective clutch 66 increases, and the hydraulic pressure67 p for the disengagement-objective clutch decreases in synchronizationwith the increase in the hydraulic pressure 59 p for theengagement-objective clutch 59 and the reduction in the hydraulicpressure 57 p for the disengagement-objective clutch 57. In this way,similarly to the time-transitional hydraulic pressure property in thefirst hydraulic type speed change unit 17′, which is shown in FIG. 17, asignificantly large common slip region is secured at the shifting-uptime in either the first hydraulic type speed change unit 17′ or thesecond hydraulic type speed change unit 20. ON/OFF operations of thepressure sensors for the individual hydraulic clutches are disclosed.

FIGS. 17 and 18 are adaptive to the primary speed change mechanism 1′.To arrange them to be adaptive to the primary speed change mechanism 1,modification is made such that, in the case of FIG. 17, the operatinghydraulic pressure 59 p for the high-speed-use hydraulic clutch 59 isreplaced with the operating hydraulic pressure 58 p for theintermediate-speed-use intermediate-speed clutch 58 in the firsthydraulic type speed change unit 17, and the hydraulic clutch 59 iscontrolled to transit at substantially in all periods. In addition, thetime-transitional voltage graph regarding the pressure sensors PSH isreplaced with the time-transitional voltage graph regarding the pressuresensor PSM, and the pressure sensors PSH is controlled to be in the OFFstate in all periods.

FIG. 18 shows the case of the speed-change with two-objective-basedhydraulic clutches being disengaged/engaged. In the primary speed changemechanism 1, since the change is performed by simply exchanging betweenthe disengagement/engagement operations of the intermediate-speedclutches 58 and 59 only in the first hydraulic type speed change unit17, it is not applicable. To arrange the case of FIG. 18 to be adaptiveto the primary speed change mechanism 1, modification is made such that,for example, the primary speed change lever 81 is shifted between thethird speed position and the fourth speed position. In this case, at theshifting-up time in the first hydraulic type speed change unit 17, thehydraulic clutch 59 is controlled to disengage, and the hydraulic clutch57 is controlled to engage. Therefore, the time-transitional graphsregarding the hydraulic pressures 57 p and 59 p, and thetime-transitional voltage graphs regarding the pressure sensors PSL andPSH, which are shown in FIG. 18, can be used without modification; andthe intermediate-speed clutch 58 transits at substantially 0 in allperiods, and the pressure sensor PSM transits in the OFF state in allperiods. In the second hydraulic type speed change unit 20, thedisengagement-objective clutches 66 and 67 are controlled to disengageand engage, and the hydraulic clutch 68 is controlled to remain in adisengaged state. Therefore, the hydraulic-pressure time-transitionalgraphs and the pressure-sensor time-transitional voltage graphs may beused without modification.

The above describes that, in the cases shown in FIGS. 17 and 18, thefluid-chamber-filling-out required periods for engagement-objectiveclutches between the hydraulic clutches 57 and 59 are not taken intoaccount for the convenience of description. In practice, however, thehydraulic-pressure time-transitional graphs take the forms, as shown inFIG. 10, which have the fluid-chamber-filling-out required periods. Thefluid-chamber-filling-out required period varies depending on whetherthe hydraulic clutches are disengaged/engaged at the time ofspeed-changing with the one-objective-based clutches or thetwo-objective-base clutches. Specifically, the fluid-chamber-filling-outrequired period increases substantially twice as much, compared to thecase of the single target set. In addition, the required period variesdepending on the engine revolution frequency. Specifically,proportionally to the reduction in the engine revolution frequency,driving forces of the hydraulic pumps are reduced. Therefore, hydraulicpressures are slowly increased to increase the fluid-chamber-filling-outrequired period.

Suppose a delay time of a disengagement start time ts with respect tothe engagement start time t₀ is fixedly set to obtain a suitable commonslip region corresponding to the operation for causing aone-objective-based hydraulic clutches to disengage/engage. In thiscase, when speed-changing with one-objective-based hydraulic clutchesbeing disengaged/engaged is performed, the pressure-rising time ta isdelayed greater than that in the former case to thereby relativelyreduce the delay time of the disengagement start time ts with respect tothe pressure-rising time ta. Therefore, the common slip region isnarrower than the common slip region that can be obtained in thespeed-change pattern for causing speed-changing with theone-objective-based hydraulic clutches being disengaged/engaged. Thatis, the area of the common slip region is small to thereby impair thespeed-change feeling. Depending on the case, the disengagement starttime ts can be earlier than the pressure-rising time ts to therebydisable a common slip region to be produced (that is, after adisengagement-objective clutch is disengaged away from a slip state, thehydraulic pressure for an engagement-objective clutch rises to cause itto be in a slip state). The incident of this kind does not conform tothe above-described basic concept.

The present invention is therefore made to compensate for the differencein the clutch-fluid-chamber-filling-out required periods in the twocases. To perform the compensation, the invention allows delay times ofthe disengagement start time ts with respect to the engagement starttime t₀ to be set differently corresponding to the individual cases.

For delay times of the disengagement start time ts with respect to theengagement start time t₀, as shown in FIG. 3 or 4, a certain number ofdelay patterns is stored in the time-setting circuit 90. According to aninput signal from a member such as the potentiometer 82 or thetachometer 83, the logical circuit 80 outputs a delay-pattern-selectionparameter to the time-setting circuit 90. A solenoid control signalbased on a selected delay pattern is input from the time-setting circuit90 to the delay circuit 88. The delay circuit 88 is used to delay anOFF-drive time of a solenoid provided to the solenoid-driver circuit 86by a predetermined amount, thereby allowing the delay time to beobtained.

In addition, corresponding to the total of four patterns at theshifting-up time and the shifting-down time in a rated-revolution stateand a low-speed-revolution state of the engine, the aforementioned delaytimes are set, and concurrently, a pressure-decrease property fordisengagement-objective clutches is set.

Specifically, the hydraulic-pressure control patterns for theprimary-speed-change hydraulic clutches are provided corresponding tofour divisional cases at the shifting-up time and the shifting-down timein the rated revolution state and the low-speed revolution state of theengine.

Hereinbelow, a description will be made regarding an embodiment shown inFIGS. 19 to 22, an embodiment shown in FIGS. 23 and 24, and anembodiment shown in FIGS. 25 and 26. In each of the figures, A shows ahydraulic-pressure control graph in the case of speed-changing withone-objective-based hydraulic clutches being disengaged/engaged, and Bshows a hydraulic-pressure control graph in the case of speed-changingwith two-objective-based hydraulic clutches being disengaged/engaged. Ineach of the figures, A and B are the same in unit-time intervals on thehorizontal axis and unit-pressure intervals on the vertical axis.

FIG. 19 shows hydraulic-pressure control states at a shifting-up time inthe rated-speed engine revolution state. The tachometer 83 shown in FIG.3 or 4 inputs a signal representative of the rated revolution state ofthe engine to the logical circuit 80. The potentiometer 82 inputs to thelogical circuit 80 a signal that represents the position of the primaryspeed change lever 81 before or after a shifting-up operation. Thelogical circuit 80 determines whether the case requires only onehydraulic clutch to be newly engaged or requires two hydraulic clutchesto be newly engaged.

FIG. 19A shows a hydraulic-pressure control state in the case whereone-objective-based hydraulic clutches are disengaged/engaged.Therefore, at a disengagement start time ts, while one hydraulic clutchis disengaged to thereby reduce an operating pressure p therefor,another hydraulic clutch remains engaged at an operating pressure pbeing kept at a normal pa. FIG. 19B shows a control state in the casewhere two-objective-based hydraulic clutches are disengaged/engaged.Therefore, at the disengagement start time ts, while two hydraulicclutches are disengaged to thereby reduce operating pressures p for thetwo clutches. In either one of the case of FIG. 19A or the case of FIG.19B, a solenoid-excitation-relief control pattern for producing th ehydraulic-pressure-decrease property graph D1 is selected in thepressure-setting circuit 89, and the operating pressure p for thehydraulic clutch to be disengaged is abruptly reduced to a level equalto or lower than the piston-holding pressure pa.

The arrangement is modified such that engine load states in the casesshown in A and B of FIG. 19 are not different from each other, and thesame solenoid-exciting pattern is set in the pressure-setting circuit 87for the both to implement the same pressure-increase pattern. Thisarrangement is common to cases shown in FIGS. 20 to 22, which will bedescribed below.

As can be seen through the comparison between FIGS. 19A and 19B, a delaytime Δt2 of the disengagement start time ts from an engagement starttime t₀, (which hereinbelow will be referred to as a “delay time”),which is shown in FIG. 19B, in the case where two-objective-basedhydraulic clutches being disengaged/engaged is set longer than a delaytime Δt1 in the case where one-objective-based hydraulic clutches aredisengaged/engaged corresponding to a time difference from theengagement start time t₀ to a pressure-rising time ta. Thereby, thestates and the areas of common slip regions shown by slanting lines in Aand B are controlled to be substantially the same. Therefore, even whenany type of shifting-up is performed, good speed-change feelings thatare similar to each other can be obtained.

FIG. 20 shows hydraulic-pressure control states at a shift-down time ina rated-engine-revolution condition, in which A shows a case whereone-objective-based hydraulic clutches are disengaged/engaged, and Bshows a case where two-objective-based hydraulic clutches aredisengaged/engaged. Similarly to the cases shown in FIG. 19, in eitherof the cases shown in A and B, a pressure-decrease pattern is set to D1.

Similarly to the cases in FIG. 19, to obtain substantially the samecommon slip region in A and B with regard to the state and the area, adelay time Δt2′ in B is controlled to be longer than a delay time Δt1′in A, corresponding to the time difference from an engagement start timet₀ to a pressure-rising time ta.

However, taking inertia generated at a time of vehicular traveling intoaccount, Δt1′ and Δt2′ at shifting-down times are controlled to beshorter than Δt1 and Δt2, respectively. Thereby, the common slip regionis reduced narrower than that at the shifting-up time shown in FIG. 19to thereby implement improvement in energy efficiency.

FIG. 21 shows hydraulic-pressure control states at a shifting-up time ina low-speed (idle revolution speed, or a revolution speed similarthereto) engine revolution. Therefore, the tachometer 83 shown in FIG. 3or 4 inputs a signal representative of the low-speed engine-revolutionstate to the logical circuit 80. Setting of delay times Δt1 and Δt2 isperformed similar to that shown in FIG. 19. However, at the low-speedengine revolution state, the revolution frequency of the hydraulic pump50 is reduced lower than that at the rated-speed engine revolution time.Therefore, the clutch fluid-chamber-filling-out required period requiredfor rise in a hydraulic pressure p to a pressure equal to or higher thanthe piston-holding pressure pa, that is, the time from an engagementstart time t, to a pressure-rising time ta is longer than that that ateach of the rated-speed engine revolution states shown in FIG. 19.Therefore, the delay times Δt1 and Δt2, which have been set taking thefluid-chamber-filling-out required period in the rated-speed enginerevolution state into account, are used without modification.Concurrently, similarly to the case shown in FIG. 19, apressure-decrease property graph is set to D1. In this case, in either Aor B, a common slip region is very narrow. That is, since a sufficientarea cannot be secured, the speed-change feeling is impaired.

In the case shown in FIG. 21, a solenoid-excitation-relieving pattern isselected in the pressure-setting circuit 89 to slowly reduce anoperating pressure for a hydraulic clutch, that is, to implement thehydraulic-pressure-decrease graph D2 or D3. To cause the pressurereduction to be slow as described above, the variable aperture Va ineach of the electromagnetic proportion selector valves is used.

According to the pressure control graph in FIG. 21A showing the case ofa time of shifting-up carried out in a state where one-objective-basedhydraulic clutches are disengaged/engaged, the pressure is abruptlyreduced at a disengagement start time. After the pressure is reduced toa specific level of the pressure, the pressure-decrease graph D2 inwhich the pressure slowly decreases is set. Thereby, the area of acommon slip region is controlled to be substantially the same as thatshown in FIG. 19.

In addition, according to the pressure control graph in FIG. 21B showingthe case of a time of shifting-up carried out in a state wheretwo-objective-based hydraulic clutches are disengaged/engaged, theabove-described fluidchamber-filling-out required period is so long thateven a disengagement start time ts set according to a delay time Δt2 setlonger than the pressure-rising time ta is earlier than thepressure-rising time ta. Therefore, the hydraulic-pressure-decreasegraph D3 in which the pressure is reduced even slower than that in thepressure-decrease graph D2 is selected to obtain a common slip region.In addition, the area of the region is controlled to be substantiallythe same as that shown in FIG. 19.

FIG. 22 shows hydraulic-pressure control states at a shifting-down timein a low-speed engine revolution. Similarly to the case shown in FIG.21, the tachometer 83 shown in FIG. 3 or 4 inputs a signalrepresentative of the low-speed engine revolution state to the logicalcircuit 80. In A and B of FIG. 22, the graphs in A and B of FIG. 21,i.e., the same pressure-decrease patterns as those in the shifting-uptimes, are set in the pressure-setting circuit 89. In the time-settingcircuit 90, similarly to the cases at the times of shifting-down carriedout in the rated-speed engine revolution state, which are shown in FIG.20, to improve the energy efficiency, delay times Δt1′ and Δt2′ that arerespectively shorter than the delay times Δt1 and Δt2 at the shifting-uptime are set. In this way, common slip regions having substantially thesame areas of the common slip regions shown in FIGS. 20A and 20B aresecured to thereby allow good speed-change feeling to be obtained.

The present embodiment has a method to obtain a constant common slipregion regardless of the time difference in thefluid-chamber-filling-out required periods of an engagement-objectiveclutch between the rated-speed engine revolution state and the low-speedengine revolution state. As can be seen through the comparison betweenFIGS. 19A and 21A, between FIGS. 19B and 21B, between FIGS. 20A and 22A,or between FIGS. 20B and 22B, to achieve the aforementioned method, theembodiment modifies the pressure-decrease property for thedisengagement-objective clutch. As an alternative method, it isconceivable that the delay time, i.e., the time between the engagementstart time t₀ and the disengagement start time, is modified. Inaddition, it is conceivable that both the pressure-decrease property anddelay time are modified.

Hereinbelow, the hydraulic-pressure control illustrated in FIGS. 23 and24 will be described. The hydraulic-pressure control shown in FIGS. 19to 22 controls the delay time to be different in the case whereone-objective-based hydraulic clutches are disengaged/engaged and thecase where two-objective-based hydraulic clutches aredisengaged/engaged. That is, in the latter case, the delay time is setto the delay time Δt2 or Δt2′. Hydraulic-pressure control shown in FIG.23 or 24, however, uses delay time Δt1 or Δt1 set either in the case ofthe speed-changing with one-objective-based hydraulic clutches beingdisengaged/engaged or in the case of the speed-changing withtwo-objective-based hydraulic clutches being disengaged/engaged.Thereby, instead of controlling the delay time to be different, thehydraulic-pressure control controls the pressure-decrease pattern to bedifferent in the individual cases.

FIG. 23 shows cases of shifting-up carried out in the rated-speed enginerevolution state. The delay time from an engagement start time t₀ to adisengagement start time ts is set to Δt1 in either the case whereone-objective-based hydraulic clutches are disengaged/engaged or thecase where two-objective-based hydraulic clutches aredisengaged/engaged. In each of the cases, asolenoid-excitation-relieving pattern suitable to the above arrangementis selected in the pressure-setting circuit 89. Thereby, as shown inFIG. 23A, in the former case, control as represented by thehydraulic-pressure-decrease property graph D1 as in the case of FIG. 19Acan be implemented; and as shown in 23B, in the latter case, control asrepresented by the pressure-decrease graph D2 including the portionwhere the pressure is slowly reduced can be implemented. Since the delaytime is set to Δt1, when two-objective-based hydraulic clutches aredisengaged/engaged, compared to the case where one-objective-basedhydraulic clutches are disengaged/engaged, a fluid-chamber-filling-outperiod (t₀ to ta) is increased to be relatively long, and the timebetween the hydraulic-pressure-rising time ta and the disengagementstart time ts is reduced relatively short. However, the pressure isinstead slowly reduced, thereby controlling the areas of the common slipregions in the cases of A and B to be substantially the same.

FIG. 24 shows cases of shifting-down carried out in the rated-speedengine revolution state. The delay time is set to Δt1′, which is shorterthan Δt1, in either one of the case where one-objective-based hydraulicclutches are disengaged/engaged or where two-objective-based hydraulicclutches are disengaged/engaged. In each of the cases, asolenoid-excitation-relieving pattern suitable to this arrangement isselected in the pressure-setting circuit 89. Thereby, as shown in FIG.24A, in the former case, -control as represented by thehydraulic-pressure-decrease property graph D1 as in the case of FIG. 20A(23A) can be implemented; and as shown in 24B, in the latter case,control as represented by the pressure-decrease graph D2 including theportion where the pressure is slowly reduced can be implemented as inthe case of FIG. 23B. Thereby, the common slip regions shown in FIGS.24A and 24B, which improves the energy efficiency, are controllednarrower than the common slip regions shown in FIGS. 23A and 23B.However, the areas of the common slip regions in the two cases arecontrolled to be substantially the same, as shown in FIGS. 24A and 24B.

The embodiment shown in FIGS. 23A and 23B discloses the control in therated-speed engine revolution state. However, control similar theretocan be implemented even in the low-speed engine revolution state. Inthis case, the delay time is controlled to be same in either, one of thecases where one-objective-based hydraulic clutches aredisengaged/engaged or where two-objective-based hydraulic clutches aredisengaged/engaged. In addition, the pressure-decrease pattern is set toallow the provision of common slip regions having the same areas as thecommon slip regions that can be obtained in the rated speed enginerevolution state. However, for example, at a shifting-up time whentwo-objective-based hydraulic clutches are disengaged/engaged, as shownin FIG. 22B, the fluid-chamber-filling-out period (t, to ta) is so longthat the common slip regions may not be obtained with the delay time Δt1′ being set. In this case, to allow the implementation of control asrepresented by a pressure-decrease graph including a pressure-decreaseslanting portion further gentler than D3 shown in FIG. 22B, it isconceivable that a solenoid-excitation-relieving pattern correspondingthereto is stored in the pressure-setting circuit 89 or that the delaytime is set longer than Δt1′ (for example, it is set to Δt2′) only forthe particular case.

FIG. 25 shows an embodiment of control at a time of shifting-up carriedout in the rated-speed engine revolution state without a delay timebeing provided. Specifically, a disengagement start time ts iscontrolled to match an engagement start time t₀. Because of thisarrangement, a solenoid-excitation-relieving pattern suitable to thisarrangement is selected in the pressure-setting circuit 89. Thereby, inthe case shown in A, control that can be represented by apressure-decrease graph (such as D2) including a portion representing aslow reduction in the pressure is implemented; and in the case shown inB, taking the fluid-chamber filling-out time is longer than that in thecase of A into account, control that can be represented by apressure-decrease graph (such as D3) including a portion representing aneven-slower reduction in the pressure is implemented. According to theabove, although no delay time is provided, common slip regions havingsubstantially the same areas as those in the cases shown in FIGS. 19Aand 19B can still be secured.

FIGS. 26A and 26B show cases at a time of shifting-up carried out in therated-speed engine revolution state, in which delay times areindividually set shorter than the delay times Δt1 and Δt2 set in thecases shown in FIGS. 19A and 19B. In each of the cases, asolenoid-excitation-relieving pattern suitable to this arrangement isselected in the pressure-setting circuit 89. Thereby, in the case shownin A, control that can be represented by a pressure-decrease graph (suchas D2) including a portion representing a slow reduction in the pressureis implemented; and in the case shown in B, taking the fluid-chamberfilling-out time is longer than that in the case of A into account,control that can be represented by a pressure-decrease graph (such asD3) including a portion representing an even-slower reduction in thepressure can be implemented. Thus, the delay time is reduced, andconcurrently, a pressure-decrease property is appropriately set.Thereby, common slip regions having substantially the same areas asthose in the cases shown in FIGS. 19A and 19B can be secured, and goodoperational feeling can be obtained.

Although the embodiments of the control shown in FIG. 25 or 26 do notdisclose a case other than that at the time of shifting-up carried outin the rated-speed engine revolution state, each of them may also beapplied to the case at a time of either shifting-up or shifting-down ina low-speed engine revolution state. In this case, the arrangement maybe made such that, while a delay time is not provided, or a delay timeis reduced, pressure-decrease properties (pressure-decrease graphs) areappropriately set to allow common slip region to be secured.Alternatively, the hydraulic-pressure control methods shown in FIGS. 19to 26 may be combined corresponding to various cases. For example, themethod may be arranged such that no delay time is provided at the timeof shifting-up in the rated-speed engine revolution state; or a shortdelay time is provided at the time of shifting-down in the rated-speedengine revolution state.

In addition, the control method may be arranged such that, as in thecase of each of the individual embodiments shown in FIGS. 19 to 22,revolution states of the engine are classified into the rated-speedrevolution state and the low-speed revolution state to varyhydraulic-pressure-decrease properties for hydraulic clutches.Alternatively, the control method may be arranged such that the enginerevolution states are not classified, but the slant degree of thehydraulic-pressure-decrease graph as shown in FIG. 11 can be variedcontinuously so as to correspond to the revolution frequencies of theengine 10, that is, so as to be less in proportion to the reduction inthe revolution frequency. For example, the method may be arranged suchthat a detection value obtained by the tachometer 83 is compared to therated engine revolution frequency, and the aperture of the variableaperture Va can be controlled corresponding to the comparison result sothat control which can be represented by a pressure-decrease graph ofwhich the slant degrees are less in proportion to the reduction in theengine revolution frequency can be implemented in the pressure-settingcircuit 89. According to this arrangement, a fluid-filling-out time fora hydraulic clutch, which is required to be longer in proportion to thereduction in the engine revolution frequency, can be continuouslycompensated for corresponding to the engine revolution frequency, and acommon slip region can be secured in the hydraulic-pressure controlgraph, thereby allowing good speed-change operation can obtained at alltimes.

Hereinbelow, a description will be made regarding detection of anabnormal hydraulic clutch, the detection being performed using thepressure sensor provided between each of the electromagnetic proportionselector valves and hydraulic clutch corresponding thereto, andregarding hydraulic-pressure control according to the detection. Apressure-increase property for an engagement-objective clutch, apressure-decrease property for a disengagement-objective clutch, andstarting times of the engagement and disengagement courses arespecifically set corresponding to required conditions. This allows theprediction to be made for time when a pressure-sensor-detecting value ofhydraulic pressure for a disengagement-objective clutch begins todecrease up to a pressure value corresponding to tolerable absorptionenergy value of a lining of a friction disc of the hydraulic clutch(switch-shifting pressure pb shown in the above-described figures suchas FIGS. 17 and 18). Therefore, time is set through the prediction ofthe pressure-decrease time. When the pressure sensors indicates a levelhigher than the aforementioned pressure value (switch-shifting pressurepb) even after the set time has passed, the logical circuit 80 receivesan input signal therefrom and thereby determines that thedisengagement-objective clutch is abnormal because of, for example,entrance of foreign substances.

As shown in the figures such as FIGS. 17 and 18, each of the pressuresensors is configured to function as a switch that turns ON whenpressure is higher than the switch-shifting pressure pb. In this case,when one of the pressure sensors for a disengagement-objective clutchstill remains in the ON state even after the above-described set timehas passed, it determines the clutch to be abnormal.

For the pressure sensor for the disengagement-objective clutch toperform the abnormality detection, it needs to identify adisengagement-objective clutch that is variable according to varioustypes of speed changes. Therefore, control therefor is complicated. Ineach of the first hydraulic type speed change unit 17 (17′) and thesecond hydraulic type speed change unit 20, a disengagement-objectiveclutch is supposed to be alternatively selected at a speed-change time.Therefore, in each of the hydraulic type speed change units, when two ormore pressure sensors are in a state higher than the switch-shiftingpressure pb (or, they are turned ON), determination can be made that thehydraulic type speed change unit includes an abnormal hydraulic clutch.Specifically, according to a calculation performed for the number ofpressure sensors that have detected hydraulic pressures higher than theswitch-shifting pressure pb after the above-described set time haspassed at a speed-change time in each of the individual hydraulic typespeed change units, determination can be made whether the hydraulic typespeed change unit is normal or abnormal without performingidentification of disengagement-objective clutches. This method may beemployed as an abnormality-determining method.

As described above, when an abnormal clutch is detected, engagementcommands for the solenoids for all the electromagnetic proportionselector valves are reset by the logical circuit 80 and thesolenoid-driver circuits 85 and 86. That is, even hydraulic clutchescommanded to engage are disengaged. The unit is thus controlled to be ina state where at most only a hydraulic clutch that cannot be disengagedbecause of a foreign substance intruded into the fluid chamber thereofis filled with operating fluid at a pressure higher than theswitch-shifting pressure. Thereby, abnormal double engagement in a geartrain is prevented.

Alternatively, it is conceivable to increase an operating pressure pthat is applied to a hydraulic clutch which is to be connected to anpressure sensor issuing an ON signal positively at the earliest time,that is, a hydraulic clutch engaged before shifting and included aforeign substance in itself to the normal pressure P₁. In this way, theaforementioned hydraulic clutch is controlled to be in acompletely-press-contacted state. Thereby, at least the foreignsubstance included in the fluid chamber of the hydraulic clutch is notsandwiched by the clutch in its engagement, and is kept in a state offloating in the fluid, thereby allowing the hydraulic clutch to beprevented from damage.

In any one of the hydraulic-pressure control methods, it is preferablethat the existence of an abnormal clutch is notified to an operator as aresult of the abnormal-clutch detection in a way of, for example,lighting a warning lamp.

FIG. 27 shows a flowchart for control according to an embodiment. Thecontrol is implemented for an instance in which a foreign substance iscarried into a hydraulic clutch. In the control, the pressure sensor isassumed to have a function as a switch that turns ON in response to thedetection of a hydraulic pressure higher than the switch-shiftingpressure pb. First, in step 401, processing determines whether or notthe engine 10 is in operation. If the engine 10 is in operation,processing proceeds to step 402. In step 402, if, in three pressuresensors in the first hydraulic type speed change unit 17 or in twopressure sensors in the first hydraulic type speed change unit 171, twoor more pieces thereof are turned ON, processing proceeds to step 403.In step 403, in order to increase the pressure for a hydraulic clutchcorresponding to a pressure sensor that has been in an ON state beforethe primary speed change lever 81 was shifted, a solenoid for anelectromagnetic proportion selector valve is excited, and othersolenoids are relieved from excitation to control the hydraulic clutchesother than the aforementioned hydraulic clutch to disengage. In short,the state of the first hydraulic type speed change unit 17 (17′) isreturned to the pre-shift state. Alternatively, the processing in thestep may be modified such that solenoids for all the electromagneticproportion selector valves in the first hydraulic type speed change unit17 (17′) are relieved from excitation. Subsequently, in step 404, awarning means (a lamp or a buzzer) for notifying abnormality caused inthe first hydraulic type speed change unit 17 (17′) is operated.

Subsequently, at step 405, in the three pressure sensors in the secondhydraulic type speed change unit 20, when two or more pieces thereof areturned ON, the control in steps 403 and 404 is performed. Also in thesecond hydraulic type speed change unit 20, if two or more pressuresensors are not turned ON, processing determines the state thereof to befree of abnormality that disables the engagement of a hydraulic clutch,allows an engagement-objective hydraulic clutch to engage, and allows adisengagement-objective hydraulic clutch to disengage (step 406).

The above almost completes intended description regarding thehydraulic-clutch hydraulic-pressure control of the present invention.Hereinbelow, a description will be made regarding an embodiment of ahydraulic circuit shown in FIG. 28, which is configured by modifying thehydraulic circuit shown in FIG. 2 to be more primitive. Theabove-described electromagnetic proportion selector valves VL, VM, VH,V1, V2, and V3 are replaced by electromagnetic selector valves VAL, VAM,VAH, VA1, VA2, and VA3, respectively. The fluid-feeder circuit 70 isconnected to these electromagnetic selector valves individually viaelectromagnetic proportion valves 110. A tank port of each of theelectromagnetic selector valves is connected to an electromagneticcontroller valve 111. Each of the electromagnetic controller valves 111is equipped with a variable aperture 111 a. In FIG. 28, the variableaperture 111 a is shown outside of the electromagnetic controller valve111 to be easily viewed. The variable apertures 111 a are providedinstead of the variable apertures Va shown in FIG. 2.

Each of the electromagnetic proportion valves 110 is set to a neutralposition N with a corresponding solenoid being set to a nonexcitationstate while it is set to an operating position I with a correspondingsolenoid excited. To cause a hydraulic clutch to be disengaged, theelectromagnetic proportion valve 110 corresponding thereto is set theneutral position N, thereby discontinuing the connection between theelectromagnetic selector valve, which is connected to the clutch, andthe fluid-feeder circuit 70. Concurrently, a solenoid for theelectromagnetic selector valve is relieved from the excitation, and isset to a fluid tank via the corresponding electromagnetic controllervalve 111. At this time, with the electromagnetic controller valve 111being set to an X position, fluid fed from the electromagneticcontroller valve 111 is returned to the fluid tank without the variableaperture 111 a being used therebetween. Therefore, there is implementedthe vertically-linear hydraulic-clutch pressure reduction at thedisengagement start time ts, which is shown in FIG. 11. With theelectromagnetic controller valve 111 being set to a Y position,operating fluid is gradually returned to the fluid tank via the variableaperture 111 a, thereby slowly reducing the hydraulic-pressure pressure.Therefore, to perform control as represented by thehydraulic-pressure-decrease property graph D1 shown in FIG. 11, wheneach of the electromagnetic proportion valves 110 and theelectromagnetic proportion valves is set to a fluid-returning position(neutral position N), the electromagnetic controller valve 111 is set tothe X position throughout the entire reduction course through which thepressure p in the fluid chamber of the disengagement-objective clutch isreduced substantially to 0. Similarly, to perform control as representedby either the pressure-decrease graph D2 or D3, when each of theelectromagnetic proportion valves 110 and the electromagnetic proportionvalves is similarly set to a fluid-returning position (neutral positionN), the electromagnetic controller valve 111 is first set to the Xposition to abruptly reduce the pressure p in the fluid chamber of thedisengagement-objective clutch, the electromagnetic controller valve 111is then switched to be set to the Y position to thereby slowly reducethe operating pressure p. In addition, the aperture of the variableaperture 111 a is adjusted to select one of the control patternsrepresented by D2 and D3.

To engage a hydraulic clutch, the solenoid for the correspondingelectromagnetic proportion valve 110 is excited to be set to a positionI, and a solenoid for an electromagnetic selector valve to be connectedthereto is also excited to control the unit to be in a state where fluidis fed from the fluid-feeder circuit 70 to the intended hydraulicclutch. In this state, the electromagnetic proportion valve 110 iscontrolled to reduce the aperture for the fluid that is fed from thefluid-feeder circuit 70 to the electromagnetic selector valve, therebyincreasing the operating pressure p that is applied to theengagement-objective clutch.

The hydraulic-circuit configuration shown in FIG. 28 can also becombined with the electrical controller circuit shown in FIG. 3 or 4 tobe used for the hydraulic-pressure control as illustrated in FIGS. 17 to27.

INDUSTRIAL APPLICABILITY

As described above, the present invention functions in the speed changemechanism having the hydraulic clutches; particularly, it functions inthe speed change mechanism configured of the plurality of hydraulic typespeed change units connected in tandem, the hydraulic type speed changeunit having the plurality of hydraulic clutches that are alternativelyengaged. The invention enables smooth, secure, and comfortablespeed-changing to be implemented at all times regardless of the enginerevolution frequency and the speed-step shift condition. In addition,the invention allows double transmission to be effectively avoided atthe time of abnormality, such as the entrance of a foreign substance inthe hydraulic clutch during the speed-changing. Therefore, the inventionprovides significant advantages for vehicles that employ the invention,such as agricultural tractors and other work tractors that require manyspeed-change steps.

What is claimed is:
 1. A method of performing hydraulic-pressure controlin a speed change mechanism comprising a plurality of speed-changinghydraulic clutches, each of which is engaged according tohydraulic-pressure-increase effects and is disengaged according tohydraulic-pressure-decrease effects, wherein a piston of each of theclutches remains neutral regardless of the quantity of suppliedhydraulic oil when a hydraulic pressure therein is less than apiston-holding pressure, and is operated at a stroke corresponding tothe quantity of supplied hydraulic oil when the hydraulic pressuretherein is not less than the piston-holding pressure, wherein while eachof the clutches that has been disengaged is engaged by supplyinghydraulic oil, the hydraulic pressure therein that has been less thanthe piston-holding pressure is gradually increased to a normalengaging-pressure above the piston-holding pressure, wherein the speedof increasing hydraulic pressure for engaging the clutch is increasedcorrespondingly to the increase of an engine revolution frequency, andwherein, during a speed-changing operation such as to disengage one ofthe clutches from an engaged state and to engage another from adisengaged state, at least either a time difference between anoperating-hydraulic-pressure-decrease start time for adisengagement-objective clutch and anoperating-hydraulic-pressure-increase start time for aengagement-objective clutch or a time-transitional decrease property ofhydraulic pressure in the disengagement-objective clutch is controlledto vary correspondingly to variations in the engine revolution frequencyso that a time-transitional pressure region where the hydraulicpressures in both the engagement-objective clutch and thedisengagement-objective clutch are equal to or more than thepiston-holding pressure is maintained to be constant regardless ofvariations in the increase-speed of hydraulic pressure in theengagement-objective clutch caused by the variations in the enginerevolution frequency.
 2. The method of performing hydraulic-pressurecontrol according to claim 1, wherein said hydraulic-pressure control isperformed according to control of an electromagnetic pressure proportionvalve provided for each of the plurality of speed-changing hydraulicclutches.
 3. The method of performing hydraulic-pressure controlaccording to claim 1, wherein, when the time difference is controlled tovary, the time difference is set longer in proportion to reduction inthe engine revolution frequency or in a case where the engine revolutionfrequency is equal to or lower than a specific level.
 4. The method ofperforming hydraulic-pressure control according to claim 1, wherein,when the time-transitional decrease property is controlled to vary, thetime-transitional decrease property is set to decrease slower inproportion to reduction in the engine revolution frequency or in a casewhere the engine revolution frequency is equal to or lower than aspecific level.
 5. The method of performing hydraulic-pressure controlaccording to claim 1, wherein the time difference is controlled to varyso that, during speed-changing, the operating-hydraulic-pressure in thedisengagement-objective clutch starts decreasing after thepiston-holding pressure arises in the engagement-objective clutch byfilling fluid in a fluid chamber of the engagement-objective clutch. 6.The method of performing hydraulic-pressure control according to claim1, wherein, regardless of variations in the engine revolution frequency,the time-transitional pressure region where the engagement-objectiveclutch and the disengagement-objective clutch commonly slip at the timeof speed-changing operation is maintained substantially to be constantaccording to the variations in the time difference and thetime-transitional decrease property.
 7. The method of performinghydraulic-pressure control according to claim 1, wherein, at the time ofspeed-changing operation, at least either the time difference betweenthe operating-hydraulic-pressure-increase start time for theengagement-objective clutch and theoperating-hydraulic-pressure-decrease start time for thedisengagement-objective clutch or the time-transitional decreaseproperty in the operating pressure for the disengagement-objectiveclutch is controlled to vary depending on whether the speed-changingoperation is a shifting-up operation or a shifting-down operation sothat, when the speed-changing operation is the shifting-down operation,the time-transitional pressure region where the engagement-objectiveclutch and the disengagement-objective clutch commonly slip is reducednarrower than that in the case of the shifting-up operation.
 8. Themethod of performing hydraulic-pressure control according to claim 7,wherein, when the time difference is controlled to vary, the timedifference at the time of the shifting-down operation is set shorterthan the time difference at the time of the shifting-up operation. 9.The method of performing hydraulic-pressure control according to claim8, wherein the time difference is controlled to vary so that, whetherthe speed-changing operation may be the shifting-up operation or theshifting-down operation, the operating-hydraulic-pressure in thedisengagement-objective clutch starts decreasing after thepiston-holding pressure arises in the engagement-objective clutch byfilling fluid in a fluid chamber of the engagement-objective clutch. 10.The method of performing hydraulic-pressure control according to claim7, wherein, if the operating-hydraulic-pressure in thedisengagement-objective clutch starts decreasing before thepiston-holding pressure arises in the engagement-objective clutch byfilling fluid in a fluid chamber of the engagement-objective clutch, thetime-transitional decrease property is varied to be gradual so that thetime-transitional pressure region where the engagement-objective clutchand the disengagement-objective clutch commonly slip is maintained. 11.The method of performing hydraulic-pressure control according to claim1, wherein tractional-load detecting means is provided in a vehicleemploying the speed change mechanism, and wherein said method modifiesat least either a time-transitional increase property in the operatingpressure for the hydraulic clutch to be engaged at the time ofspeed-changing or the time-transitional decrease property in theoperating pressure for the hydraulic clutch to be disengaged at the timeof speed-changing depending on whether or not said tractional-loaddetecting means detects a tractional load.
 12. The method of performinghydraulic-pressure control according to claim 1, wherein a governormechanism capable of performing control of an engine revolutionfrequency according to detection of an engine load is provided in avehicle employing said speed change mechanism, and wherein said methodmodifies at least either a time-transitional increase property in theoperating pressure for the hydraulic clutch to be engaged at the time ofspeed-changing or the time-transitional decrease property in theoperating pressure for the hydraulic clutch to be disengaged at the timeof speed-changing depending on whether or not said governor mechanismdetects an engine load equal to or higher than a specific level.
 13. Themethod of performing hydraulic-pressure control according to claim 1,wherein the plurality of hydraulic clutches are classified and allocatedin a plurality of hydraulic type speed change units connected in tandem,and the hydraulic clutches are alternatively engaged in each of saidhydraulic type speed change units to thereby form one speed-change step.14. The method of performing hydraulic-pressure control according toclaim 13, wherein pressure-detecting means is provided to detect anoperating hydraulic pressure for each of the hydraulic clutches, andwhen two or more units of said pressure-detecting means detects ahydraulic pressure higher than a specific pressure value in one of saidplurality of hydraulic type speed change units, one of twohydraulic-pressure control steps is performed, one hydraulic-pressurecontrol step being performed to engage only those of the hydraulicclutches which have immediate-previously been disengaged, and the otherone hydraulic-pressure control step being performed to disengage all thehydraulic clutches.
 15. The method of performing hydraulic-pressurecontrol according to claim 14, switches being provided as saidpressure-detecting means, wherein each of said switches turns ON or OFFwith respect to the border of the specific pressure value to therebydetermine whether the operating hydraulic pressure of corresponding oneof the hydraulic clutches is higher or lower than the specific value.